Why is my computer running slow? 10 common causes (and how to fix them).

Computers rarely become slow overnight without a reason. Something changed: a new program added itself to startup, the drive quietly filled up, an update is waiting to finish, or dust has built up inside the vents. The machine hasn't failed, it's working harder than it should for the same everyday tasks.

In our experience, almost every slow computer traces back to one of a small handful of causes: too many startup programs, not enough free storage, overheating, outdated software, unwanted software or malware, and occasionally hardware that has simply reached the end of its useful life.

The good news is that most of these can be fixed without replacing the computer, and several take only a few minutes. Work through the ten causes below in order; they're arranged from the most common and easiest to check, to the ones that need a more honest conversation about cost.

1. Too many startup programs.

This is the single most common reason a computer running slow feels worse over time. Every printer utility, chat app, photo tool and updater you install tends to add itself to startup, so it launches automatically the moment you sign in.

The result is a long boot time and a machine that stays busy for several minutes after the desktop appears. Each program on its own is small; twenty of them together are not.

How to check startup apps.

• Windows 11 and 10: open Settings, then Apps, then Startup. You'll see every app that launches at sign-in, with a rough impact rating.
• You can also press Ctrl + Shift + Esc to open Task Manager and choose the Startup apps tab for the same list.
• Mac: open System Settings, then General, then Login Items, and review the list of items that open at login.

How to disable them safely.

Turn off anything you recognise and don't need immediately at sign-in, music apps, games launchers, photo uploaders, printer dashboards, shopping toolbars. Disabling a startup item doesn't uninstall the program; you can still open it whenever you want.

Leave anything you don't recognise alone, and leave your security software and touchpad, audio or graphics utilities enabled. If a name means nothing to you, look it up before changing it rather than guessing.

2. Running low on storage.

A nearly full drive slows a computer down in ways that surprise people. Windows and macOS both need free space to work in for temporary files, updates, and virtual memory when RAM runs short.

On an SSD, performance drops noticeably once the drive is close to full because the drive has less room to organise data efficiently. On an older mechanical hard drive (HDD), files also become fragmented and take longer to read.

Where the space usually goes.

• Temporary files use Windows Storage Sense (Settings, System, Storage) or Disk Cleanup rather than a third-party cleaner.
• The Downloads folder, which quietly collects installers, PDFs and photos for years.
• The Recycle Bin or Trash, which still occupies space until it's emptied.
• Applications you no longer use, including trial software that shipped with the computer.
• Large video files, old backups and duplicate photo libraries.

As a practical rule, aim to keep 20–25% of your drive free. If you can't get there by clearing files, an external drive or a cloud service is a better answer than deleting things you may need.

3. Too many browser tabs.

Browsers are the heaviest program most people run. Chrome, Edge, Firefox and Safari each keep open tabs loaded in memory so they respond instantly, which is wonderful with eight tabs and painful with eighty.

If your computer is lagging only while browsing typing feels delayed, the fan spins up, other apps become sluggish tabs are usually the cause rather than the computer itself.

• Close tabs you've finished with, and bookmark the ones you're keeping 'just in case'.
• Review your extensions and remove any you didn't deliberately install; extensions run constantly, and coupon or 'search helper' add-ons are a frequent culprit.
• Restart the browser occasionally, it releases memory that builds up over days of use.

4. Outdated Windows or macOS.

Updates aren't only about new features. They include security patches, performance improvements and driver updates that keep your graphics, storage and network hardware working properly.

A computer waiting on a large pending update often feels slow while it downloads and installs in the background — and stays slow until it's allowed to restart and finish.

• Windows: Settings, then Windows Update, then Check for updates. Install what's offered and restart when asked.
• Mac: System Settings, then General, then Software Update.
• Get drivers from Windows Update or the manufacturer's own support site only. Never install a 'driver updater' utility.
• Restart properly after a major update — closing the lid isn't the same thing.

5. Not enough RAM.

People often mix up RAM and storage. Storage is where your files live permanently; RAM is the working space your computer uses for whatever is open right now. When RAM runs out, the computer starts shuffling data to the drive instead, and everything slows to a crawl.

Common symptoms of low RAM.

• Switching between programs takes several seconds.
• The computer is fine with one or two apps but stalls with several open.
• Video calls stutter while a browser and email are also running.

Adding RAM helps when the computer is otherwise healthy, a reasonable processor, an SSD, and 4GB or 8GB of memory doing too much. It won't help if the real problem is a failing drive, malware, or overheating, and many modern laptops have memory soldered in place and can't be upgraded at all. That's worth confirming before spending anything.

6. Malware or unwanted software.

Not every performance problem is a virus, and there's no need to panic. But unwanted software is a genuine cause of a slow PC, and it usually arrives bundled with something that looked helpful.

Warning signs.

• Pop-ups warning that your computer is 'infected' or 'needs cleaning'.
• Your browser homepage or search engine changed on its own.
• New toolbars, extensions or programs you didn't install.
• The fan running constantly while the computer sits idle.

Fake cleaners, fake antivirus products and browser hijackers all rely on urgency to get installed. Windows already includes Windows Security (Virus & threat protection) open it and run a full scan. On a Mac, keep macOS updated and remove browser extensions you don't recognise. If you prefer a paid product, buy a well-known one directly from its maker; never from a pop-up.

7. The computer is overheating.

Heat is the quiet cause people rarely check. When a processor gets too hot it deliberately slows itself down to protect the hardware, a behaviour called thermal throttling. The computer still works; it just runs at a fraction of its normal speed.

• Dust in the vents and fans is the usual reason, especially after two or three years.
• Laptops used on beds, sofas and blankets block their own air intakes underneath.
• Desktops pushed tight against a wall or into a cabinet have nowhere to exhaust warm air.

Signs to look for: the case or keyboard is hot to the touch, the fan is loud constantly, and performance drops after ten or fifteen minutes of use rather than immediately. Use the computer on a hard, flat surface, keep vents clear, and have dust cleaned out every few months. Internal cleaning on a laptop is worth having done carefully rather than forced.

8. An aging hard drive.

If your computer is more than about six years old, it may still use a mechanical hard drive (HDD) with spinning platters. Replacing it with a solid-state drive (SSD) is the single biggest improvement most older computers can get, often the difference between a two-minute startup and a fifteen-second one.

Signs a drive is struggling.

• Files and folders take a long time to open, even small ones.
• The computer freezes for several seconds at a time, then recovers.
• Clicking, grinding or repetitive whirring sounds.
• Occasional errors about files that can't be read.

Noises and read errors deserve immediate attention: copy your important files somewhere safe first, then investigate. An SSD upgrade is usually worthwhile on a computer that's otherwise sound, and rarely worth it on one that already struggles in every other respect.

9. Too many background applications.

Beyond startup programs, plenty of software keeps working after you've closed its window. Individually they're reasonable; together they consume a slow desktop's entire capacity.

• Cloud syncing - OneDrive, iCloud, Dropbox and Google Drive uploading a large folder in the background.
• Game launchers downloading updates without being opened.
• Backup software running mid-afternoon instead of overnight.
• More than one antivirus program installed, each scanning the other.

To see what's actually busy, open Task Manager on Windows (Ctrl + Shift + Esc) or Activity Monitor on a Mac, and sort by CPU and then by Memory. If one program sits at the top consistently, you've found your answer. Pause syncing while you work, schedule backups overnight, and keep a single security program.

10. Old hardware that has reached its limit.

Sometimes the honest answer is that the hardware has done its work. A computer built eight or ten years ago was designed for the software of its time, and no amount of tidying changes what it's physically capable of.

The question then becomes financial rather than technical:

1. 01Repair makes sense when one component has failed, a drive, a battery, a fan — and the rest of the computer is fine.
2. 02Upgrade makes sense when a modest amount (an SSD, extra memory) buys you two or three more comfortable years.
3. 03Replacement makes sense when the cost of upgrades approaches a third or more of a new computer's price, or when the machine can no longer run a supported version of Windows or macOS.

A useful test: if a computer is slow at everything, all the time, even after clearing storage and startup programs, it's usually hardware. If it's slow only sometimes, or only in one program, it's usually something fixable.

A quick performance checklist.

Most households never need to think about computer optimization again if they keep to this short routine:

• ✓ Restart the computer at least once a week.
• ✓ Keep Windows or macOS updated.
• ✓ Remove software you no longer use.
• ✓ Keep 20–25% of your storage free.
• ✓ Empty the Downloads folder and Recycle Bin regularly.
• ✓ Run a full malware scan once a month.
• ✓ Have dust cleaned from vents and fans every few months.
• ✓ Restart properly after every major update.

When it's worth getting help.

There's no prize for spending a weekend on this. Secure online support is often the faster and cheaper route when:

• The computer is still slow after you've worked through the steps above.
• You're seeing blue screens, freezes or unexpected restarts.
• You need files moved from an old computer to a new one safely.
• You suspect a virus, or something has changed your browser settings.
• You're considering an SSD or memory upgrade and want an honest opinion first.
• Windows needs reinstalling and you'd rather not lose anything.
• A printer or scanner stopped working after an update.
• Wi-Fi or network trouble is being mistaken for a slow computer.

A specialist can look at the machine with you, in a session you can see and end at any time, and tell you plainly what's causing it, including when the answer is that nothing needs buying.

“Nine times out of ten, a slow computer isn't asking to be replaced. It's asking for twenty minutes of attention.”