My computer is slow: 7 reasons and how to fix them

A start-up that takes three minutes, a browser that lags the moment you open five tabs, a fan that roars during your coffee break, applications that take ten seconds to open: if your computer has these symptoms, you're not alone. Slowness is one of the most frequent complaints I get — and the good news is that in 70% of cases, the culprit isn't your hardware. It's the build-up of little things that ends up smothering the machine.

Here are the 7 most common causes, from the easiest to fix to the most technical. For each one, I explain the problem, how to tell if it applies to you, and the solution.

1. Too many startup programs

Over the course of installations, a lot of software adds a little agent that launches automatically when you start your PC or Mac: Skype, Teams, Adobe Creative Cloud, Spotify, OneDrive, multiple antivirus programs, update agents for Java, Razer, Logitech, and so on. Taken one at a time, it's harmless. With 25 startup programs, your machine takes three minutes before it's usable.

How to tell: On Windows, open the Task Manager (Ctrl+Shift+Esc) → Startup tab. You'll see the whole list with their estimated impact. If you have more than 15 programs set to "Enabled", you have a problem.

Solution: Disable everything that doesn't need to launch automatically. Skype, Teams, Spotify: no, you'll open them when you need them. OneDrive if you use it, yes. Antivirus, yes. Everything else, generally no. On Mac: System Settings → General → Login Items (or Login Items on older versions).

2. A mechanical hard drive instead of an SSD

If your PC or Mac dates from before 2018-2019, there's a good chance it still has a mechanical hard drive (HDD) and not an SSD. This difference is the main cause of slowness on machines that are a few years old. An HDD reads data at around 100 MB/s. A modern SSD: 500 to 3500 MB/s. That's between 5 and 35 times faster.

How to tell: On Windows, open the Task Manager →Performance tab → Disk. If it says "HDD" or "Mechanical drive", that's it. On Mac: Apple menu → About This Mac → Storage. If you see "Hard drive" instead of "Flash Storage" or "SSD", that's it.

Solution: Replacing the hard drive with an SSD, with an identical clone of the system. It's the upgrade with the best return on investment for a computer that's lagging. For €100 to €180 (part + labour), your machine regains a modern speed. Many of my clients extend the life of their PC or Mac by 4-5 years with this single change.

3. Not enough RAM (memory)

RAM is your computer's working memory. When you open a program, it loads into it. If you don't have enough RAM for what you do, your machine is forced to use the disk as backup memory — which is up to 100 times slower. The result: everything lags, especially when you have several tabs or applications open.

How to tell: Task Manager → Performance → Memory. If you're constantly above 80% RAM usage during normal use, you're short on it. For typical use in 2026, consider 8 GB a strict minimum, 16 GB comfortable.

Solution: Adding RAM if the machine allows it (most desktop PCs and some laptops, almost no recent MacBook where it's soldered on). Reckon on €30 to €80 to go from 8 to 16 GB. On recent Macs with soldered memory, there's no choice: you have to plan for that amount of RAM at the time of purchase.

4. Junk software and adware

Over the course of installations, your PC accumulates, without you knowing: browser toolbars, "optimisers" that do nothing, extensions that hijack searches, programs that slipped in through "free" installers (the infamous pre-ticked boxes). This software isn't made up of viruses in the strict sense — antivirus programs often let it through — but it slows the machine down considerably.

How to tell: If you see advertising pop-ups outside the browser, if your default search engine has changed on its own, if you have several toolbars in Chrome or Edge, if unknown programs open at start-up — that's it.

Solution: A thorough clean-up with Malwarebytes (free, far more effective than standard antivirus programs against this kind of threat), manual uninstallation of dodgy browser extensions, restoring default settings. That's what I do during an optimisation visit.

5. A paid antivirus that's too heavy

This is a touchy subject: many people think that having a "real" paid antivirus (Norton, McAfee, Avast Premium, Bitdefender, Kaspersky…) protects them better than a free one. In reality, on Windows 10 and 11, Windows Defender (built-in, free) is enough for 95% of use cases — it has detection rates comparable to paid antivirus programs in independent tests such as AV-TEST.

But above all: paid antivirus programs weigh very heavily on performance. They constantly scan every file you open, every web page, every email. On an older machine, that translates into visible slowdowns.

Solution: Uninstall your paid antivirus cleanly (with their official uninstall tool, not just "Add/Remove"), enable Windows Defender, and complement it with the free Malwarebytes for monthly manual scans. A saving of €50-80/year and a faster machine.

6. A full hard drive

When your disk approaches 90% full, the operating system struggles to manage temporary files, updates and the swap (backup memory). The result: gradual slowdown, applications that crash, Windows updates that fail.

How to tell: On Windows, open File Explorer → This PC. If the bar for the C: drive is red or almost full, there's a problem. On Mac: Apple menu → About This Mac → Storage.

Solution: Have a clear-out — uninstalling unnecessary programs, deleting old downloads, moving photos/videos to an external drive or the cloud, emptying the bin (often forgotten), cleaning up Windows temporary files (the built-in "Disk Clean-up" tool). Ideally, keep at least 20% of the disk free.

7. A worn-out operating system

After several years without a reinstall, Windows accumulates leftover configurations, orphaned registry entries, obsolete drivers and remnants of uninstalled software. It's less common than it used to be (Windows 10 and 11 are more robust than XP or 7), but it happens — typically on machines set up 5+ years ago and updated through successive versions of Windows.

How to tell: If, after trying all the previous solutions, the machine stays slow and Windows errors crop up regularly — it's probably this.

Solution: A clean reinstall of the system with a prior backup of your data. More drastic but very effective: you get brand-new-machine performance back. Reckon on €100 to €150 for a complete job (backup, reinstall, restoring your data and essential programs).

Which solution for your case?

If you've read this article this far, you've probably already identified 1 or 2 causes that ring true for you. Here's the order I recommend in 95% of cases:

1. Do the free clear-out in 30 min: disable startup programs (cause 1), clean up the disk (cause 6), replace the paid antivirus with Defender + Malwarebytes (cause 5).
2. If it's still slow, check the hardware: do you have an SSD? enough RAM? If not, the SSD upgrade (cause 2) is the best investment.
3. If it's still slow: a thorough clean-up by a pro (junk software, cause 4) or a complete reinstall (cause 7).

The logic: start with what's free and reversible, finish with what's costly and drastic. In the vast majority of cases, steps 1 and 2 are more than enough.