A computer is slow when one of its resources runs short: memory (RAM), storage speed, free disk space, processor time or cooling. The usual causes are more apps and browser tabs than the installed RAM can hold, Windows on a hard disk drive, a nearly full drive, background updates, heat and malware. Task Manager or Activity Monitor shows which one it is.
This guide explains why each cause slows a PC or a Mac and how to confirm it with the tools built into Windows 11 and macOS, with menu names as of September 2026 (Windows 11 version 25H2 and macOS 27). Each cause ends with the fix in a line or two and a link to the guide that owns it: Windows settings tuning, hardware upgrades, diagnosis by symptom and a 15-minute checklist of quick wins. If the computer crashes, will not start or loses Wi-Fi or sound rather than being slow, see troubleshooting common PC problems.
The nine causes at a glance
Microsoft's own summary of why a Windows device runs slowly names limited storage space, too many startup apps running in the background, outdated software and hardware that no longer meets modern performance needs. Apple's page on a Mac that runs slowly starts with the same two suspects: not enough free space on the startup disk, and an app that needs more memory than the Mac has available. Match what you see to a row, then read that section.
| Symptom | Likely cause | How to confirm | Where the fix is |
|---|---|---|---|
| Slower with every app or tab you open | Not enough memory | Task Manager > Performance > Memory: Committed near its limit | Settings or more RAM |
| Slow to start and to open anything, even with RAM free | A hard disk drive | Settings > System > Storage > Advanced storage settings > Disks & volumes | Move to an SSD |
| Low disk space warnings, updates that will not install | A nearly full system drive | File Explorer > This PC; on a Mac, General > Storage | 15-minute checklist |
| Slow for several minutes after you sign in | Startup and background apps | Task Manager > Startup apps, Startup impact column | 15-minute checklist |
| Only the browser is slow | Tabs and extensions | The browser's own task manager, sorted by memory | Diagnosis by symptom |
| Slow for hours after an update or on a new PC, then fine | Updates and indexing | Settings > Windows Update; the search indexing status | Let it finish, plugged in |
| Quick at first, then slower; fans loud, case hot | Thermal throttling | Slows under sustained load; better on a hard, cool surface | Clean vents and fans |
| Busy when you are doing nothing; unexpected ads | Malware or unwanted apps | A Quick scan in Windows Security | Choose protection |
| Slow at everything, even after all of the above | Ageing hardware | Settings > System > About; PC Health Check | Upgrade or replace |
Tip
Check while the computer is slow, not after a restart has cleared it. Press Ctrl + Shift + Esc for Task Manager, or open Activity Monitor on a Mac, and sort the process list by CPU, then memory, then disk. The process at the top of each column is your first lead.
Not enough memory: RAM pressure and paging
RAM is the computer's short-term working space. Microsoft's explainer on memory puts it plainly: apps and files are loaded into RAM from slower disks so they can be reached faster, and the more memory a device has, the more it can do at the same time without slowing down. Every open app, browser tab and background service takes a share.
When the shares add up to more than the installed RAM, the operating system does not refuse; it makes room. Windows moves memory pages that are accessed infrequently out to the page file on the disk. A Mac compresses the memory of inactive apps and swaps unused data to the startup disk. Either way, the next time you switch to that app its pages come back from the disk, which is slower than RAM, and on a hard disk drive very slow (see the next section). That is RAM pressure: the computer still works, but every switch between apps waits for the drive.
Windows also keeps count of what it has promised. The commit charge is the memory promised to all programs; the commit limit is installed RAM plus the page file. Microsoft warns that if the charge reaches the limit, the system and programs may not get the memory they ask for, which can cause freezing and crashing.
How to confirm a memory shortage
On Windows, open Task Manager, select Performance > Memory and read Committed: the first number is the commit charge, the second the commit limit. Then sort the Processes page by Memory to see who is using it. Microsoft's performance troubleshooting guide gives thresholds for the underlying counters:
- % Committed Bytes In Use: up to 50% is healthy, 60 to 80% is a warning and 80 to 100% is critical.
- Available MBytes: more than 10% of RAM, or at least 4 GB, free is healthy; below 1% or 500 MB is critical.
To watch those counters for a minute, together with the disk latency counters from the next section, run this in PowerShell. Get-Counter takes a sample every 5 seconds, 12 times:
Get-Counter -Counter '\Memory\% Committed Bytes In Use','\Memory\Available MBytes','\PhysicalDisk(*)\Avg. Disk sec/Read','\PhysicalDisk(*)\Avg. Disk sec/Write' -SampleInterval 5 -MaxSamples 12
On a Mac, open Activity Monitor and select Memory. The Memory Pressure graph is green when the Mac is using its RAM efficiently, yellow when it might eventually need more, and red when it needs more RAM. Swap Used shows how much memory is sitting on the startup disk.
Note
Little free memory is not a problem by itself. Apple points out that free memory does not necessarily make a computer faster, because macOS performs best when it uses and manages all of the memory. What matters is paging: committed memory near its limit, or yellow and red memory pressure.
The fix: close what you are not using, and trim what starts by itself. If your normal day keeps committed memory near the limit, you need more RAM: Microsoft recommends 8 GB to keep a device running well long term, and 16 GB or more for photo and video editing. Our hardware upgrade guide shows how to check for free memory slots, and the settings guide explains why a bigger page file is not the answer.
A hard disk drive instead of an SSD
A hard disk drive (HDD) stores data on spinning platters. A read-write head on a moving arm has to swing to the right track and then wait for the right spot on the platter to come round. Microsoft's performance guide does the arithmetic for a typical hard disk spinning at 5,400 revolutions per minute: one turn takes about 11 ms, so reading an arbitrary block can take that long. Starting Windows or opening an app means many small reads from different places on the drive, and each one waits.
A solid-state drive (SSD) keeps data in flash memory chips, with nothing to spin or move. Microsoft's guide to storage types describes SSDs as smaller and faster than hard disk drives, and HDDs as more common in older devices. A PC with Windows on a hard disk drive is slow to start, slow to open apps and slow to update, however much RAM it has. It also makes a memory shortage much worse, because every page moved out to the page file lands on the slowest part of the system.
How to confirm a hard disk drive
Go to Settings > System > Storage > Advanced storage settings > Disks & volumes. Windows lists each disk's type, SSD or HDD, with its capacity. Then check whether the drive is the bottleneck: in Task Manager, sort Processes by Disk, and for a measured answer run the PowerShell command above. The disk counters are in seconds, so 0.025 is 25 ms. Microsoft's guide treats an average read or write time under 15 ms as healthy, above 25 ms as a warning and above 50 ms as critical, when it lasts a minute or more.
Fragmentation is rarely the problem on a modern PC. Windows optimizes drives once a week by default: hard disk drives are defragmented, and SSDs are trimmed instead. A manual defragment will not make a hard disk drive behave like an SSD.
The fix: move Windows onto an SSD. Our hardware upgrade guide covers SATA and NVMe drives, and cloning the old drive or installing Windows clean.
A system drive that is almost full
Microsoft says that a PC running out of storage space may show slower performance or trouble installing Windows updates. Three mechanisms are behind that:
- The page file cannot grow. A system-managed page file grows when the commit charge reaches 90% of the commit limit, up to three times the RAM or 4 GB (whichever is larger, but no more than an eighth of the volume). Microsoft's page file documentation adds the condition: enough free disk space must be available. On a full drive, memory pressure turns into freezing sooner.
- Updates need room. Windows reserves part of the drive for temporary files, caches and updates, and clears that reserve when space runs low so an update can still install. Once the reserve is gone, there is no margin for the next one.
- An SSD needs empty blocks. Flash memory is written in pages but erased in whole blocks, and a page can be written directly only when it is empty. Kingston explains that keeping blocks empty in reserve keeps an SSD's performance consistent, and that TRIM lets the drive reclaim unused capacity for that reserve. A drive with almost no free space has less to work with.
How to check free space
On Windows, open File Explorer and select This PC: the free space on each drive appears under Devices and drives. Settings > System > Storage breaks the used space into categories. On a Mac, choose Apple menu > System Settings > General > Storage; Apple's guide to freeing space explains each category, including System Data.
The fix: remove temporary files and apps you do not use, and move large files elsewhere. The 15-minute checklist does the quick pass, and the settings guide sets up Storage Sense so it stays done.
Too many startup and background apps
Many apps register themselves to start when you sign in: sync clients, chat apps, updaters, printer and audio utilities. Microsoft notes that apps that run automatically at startup affect both how fast the device starts and how the system performs afterwards, and its performance tips add that some apps run background processes even when you are not using them. Each takes a share of memory, and many wake up to check for updates or sync, which costs processor and disk time.
How to see what starts with the computer
Open Task Manager and select Startup apps. The Startup impact column rates what each app cost at the last start-up: High means more than 1 second of processor time or more than 3 MB of disk activity, and Low means less than 300 milliseconds and less than 292 KB. The same list, with a switch per app, is in Settings > Apps > Startup.
On a Mac, open Apple menu > System Settings > General > Login Items & Extensions. The list of login items shows what opens at log-in, and a switch per app controls whether it may run in the background, for example to check for software updates or sync data.
The fix: disable what you do not need the moment you sign in; you can still open it yourself. The 15-minute checklist walks through it.
Browser tabs and extensions
Each open tab is a live page with its own scripts and memory, and each extension adds code of its own. Google's advice for Chrome is direct: the more tabs you have open, the harder Chrome has to work, and turning off extensions you do not use speeds it up. Microsoft built sleeping tabs into Edge for the same reason: by default it puts background tabs to sleep after two hours of inactivity to free resources for active tabs and other applications, which means an idle tab holds on to them until then.
How to find the heavy tab or extension
In Chrome, select More > More tools > Task Manager and sort by Memory. Google suggests looking for items labelled "Background page". If only the browser is slow while other apps stay quick, the cause is the browser, its extensions or the network, not the computer; our diagnosis guide separates those.
The fix: close tabs you are not using, remove extensions you do not need, and let the browser put idle tabs to sleep. The 15-minute checklist covers sleeping tabs in Edge and Memory Saver in Chrome.
Updates and indexing running in the background
Some slowness is necessary work that ends by itself. Windows 11 downloads and installs updates automatically, and tries to restart to finish them when you are not using the device. Cumulative security updates arrive on the second Tuesday of each month, and a feature update once a year in the second half of the year. On home and unmanaged business PCs, Windows Update also starts a feature update by itself as a version approaches the end of servicing. For Home and Pro editions, version 24H2 reaches its end of updates on October 13, 2026, so PCs still on it are being moved to a newer version around now.
Search indexing is the other big one. The first full index can take up to a couple of hours; after that, Windows only re-indexes files that change. Enhanced mode indexes the entire PC and may use more resources than Classic. Copilot+ PCs add semantic indexing, which is on by default, and Microsoft recommends keeping them plugged in until the first index completes. A full antivirus scan checks every file and program, so it keeps the disk busy until it ends.
Macs do the same work. Apple notes that the fans run faster while Spotlight indexes the drive after you migrate data, and Software Update can download and install macOS updates automatically.
How to tell background work from a fault
The pattern gives it away: a new PC, a big update or a data migration, then hours of a busy disk, and afterwards the computer is fine. Open Settings > Windows Update to see whether an update is downloading, installing or waiting for a restart; Update history shows what was installed and when. Microsoft points to Settings > Privacy & security > Searching Windows for the indexing status. In Task Manager, the busiest processes are usually Windows' own rather than an app's.
The fix: leave the computer on, plugged in and connected until the work finishes, and restart when Windows asks. If the disk never settles, the diagnosis guide shows how to find what keeps it busy; our guide to keeping a computer updated covers updating safely.
Heat: thermal throttling from dust or a failing fan
Processors protect themselves from heat. Intel describes throttling as a mechanism that reduces the clock speed when the temperature rises above the processor's maximum (TjMax), both to protect the processor and to signal that the system is overheating. The symptoms Intel lists are a slower processor and stuttering in games. Its checklist is all about cooling: a fan that works, air that can get in and out, and a cooler fitted correctly with the right amount of thermal paste. On laptops, the manufacturer sets the power limits, so how a laptop throttles depends on how its maker designed it.
Dust is the usual culprit on an older machine. It clogs the intake vents and the cooling fins, and a failing fan moves less air, or none. Less air through the case means the processor reaches its limit sooner and stays there longer. Where the computer sits matters too: Apple asks for an ambient temperature between 10 and 35 °C for its laptops, and notes that on a soft surface such as a couch, a bed or your lap, the fans might run more.
How to recognize throttling
The pattern is time-based: a heavy task starts at full speed, then slows as the fans spin up and the case gets hot, and it runs better on a hard, cool surface. A fan that stays silent under load, grinds or rattles is a hardware fault to repair. On a Mac laptop that gets warm without heavy work, Apple suggests opening Activity Monitor and checking CPU activity for a process that is loading the processor. Apple also warns that third-party temperature apps do not measure the case temperature and should never be used to diagnose hardware.
The fix: clear the vents and fans, and replace a failing fan. Our guide to cleaning computer hardware safely covers vents, fans and when to replace thermal paste.
Malware and unwanted software
Microsoft's performance tips note that malware can consume CPU and disk resources, and some of it exists only to burn processor time: a coin miner runs the calculations that generate cryptocurrency, which requires significant computing resources, on someone else's hardware. Below malware sits a grey zone: Microsoft describes potentially unwanted applications as software that can make a machine run slowly, show unexpected ads or install other software.
How to check for malware
Suspect it when the computer is busy while you are doing nothing, when an unfamiliar process keeps returning to the top of Task Manager, or when ads and toolbars appear that you did not install. Open Settings > Privacy & security > Windows Security > Virus & threat protection and run a Quick scan. If something is found, or the computer still seems wrong, run Microsoft Defender Antivirus (offline scan) from Scan options: it restarts the PC and scans without loading Windows, where persistent malware has a harder time hiding.
The fix: remove what the scan finds, then close the door it came in through. Our guides to protecting a computer from malware and choosing antivirus software cover both, including whether the built-in Microsoft Defender is enough.
Hardware that is older than the software
Software requirements rise and hardware stays the same. The minimum requirements for Windows 11 are a 1 GHz or faster processor with 2 or more cores that appears on Microsoft's list of approved CPUs, 4 GB of RAM, a 64 GB or larger storage device, UEFI firmware capable of Secure Boot, and TPM 2.0, plus DirectX 12 graphics and a 720p display. Microsoft points out that the processor's clock speed and core count are not upgradable, while RAM and storage sometimes are.
A PC at those minimums runs Windows 11, but not comfortably: the 4 GB floor is the same figure Microsoft gives for light browsing and documents, while it recommends 8 GB for the long term. Microsoft's performance page is candid about the limit of tuning, too: older PCs with outdated hardware may not see significant improvements, and upgrading the device may be the best long-term solution. Windows 10 support ended on October 14, 2025, so a PC that cannot move to Windows 11 gets security fixes only through the consumer Extended Security Updates program, which Microsoft says covers it until October 12, 2027.
Macs follow the same curve. Apple's compatibility list for macOS 27, published in September 2026, contains only Macs with Apple silicon, so Intel-based Macs stay on earlier versions. Apple also suggests quitting any app that is not compatible with the Mac, for example one that requires a different processor or graphics card.
How to check the hardware against the requirements
On Windows, go to Settings > System > About. Under Device specifications you see the processor and the amount of RAM installed; compare them with the requirements above and with those of the apps you use most. On a Windows 10 PC, the PC Health Check app checks Windows 11 eligibility and says why a device does or does not qualify. On a Mac, choose Apple menu > System Settings > General > About.
The fix: upgrade the part that limits you (usually RAM or the drive) if the rest of the machine is sound, or replace the computer when the processor or the missing TPM 2.0 rules out a supported system. Our hardware upgrade guide helps you decide.
When it is more than one cause
Causes compound. A laptop with 4 GB of RAM and a hard disk drive pages memory to the slowest drive it has; a nearly full drive stops the page file from growing when memory runs short; a dusty laptop throttles just as the first search index starts. Work through them in order: confirm which resource is short, fix the cheapest cause first, then measure again with the same view in Task Manager or Activity Monitor so you know the change worked.
When slow computers are a recurring ticket across a team, the cause is rarely one machine. Our IT support service traces recurring issues to their cause and fixes them at the root, patches operating systems and apps on a schedule, and keeps an asset register with advice on what to replace and when.
