The most common reasons your computer slows down

Computer lag usually comes from one of four sources: too many programs running at once, a hard drive that is nearly full, malware or viruses, or hardware that is straightforward old. Your machine slows down because it is working harder than it was designed to, or because something is using resources in the background without your knowledge. The good news is that most lag has a fix you can do yourself in under an hour.

The first step is to figure out which of these four is actually happening on your machine. A computer that lags only when you open certain programs (like video editing software or games) is usually hitting the limits of its RAM or graphics card. A computer that lags all the time, even when you are not doing anything, usually has a full hard drive, malware, or a failing hard drive that is struggling to read files.

Key Takeaways

  • Lag usually comes from too many background programs, a nearly full hard drive, malware, or aging hardware — and you can narrow down which one in a few minutes.
  • Open Task Manager on Windows (Ctrl+Shift+Esc) or Activity Monitor on Mac (Command+Space, then type Activity Monitor) to see which programs are using the most CPU and memory right now.
  • A hard drive that is more than 85 percent full will slow down noticeably, even if nothing is running — delete old files or move them to external storage to free up space.
  • Malware and viruses often hide in the background and use your processor and internet connection; run a full scan with Windows Defender (Windows) or Malwarebytes to check for infections.
  • If your computer is more than five or six years old and you have ruled out the other causes, the hardware itself may be wearing out and a technician can test it.

Check what is running in the background right now

The fastest way to find lag is to look at what your computer is actually doing at this moment. On Windows, press Ctrl+Shift+Esc to open Task Manager. On Mac, press Command+Space, type "Activity Monitor", and press Enter. Both tools show you every program that is running and how much of your processor (CPU) and memory (RAM) each one is using.

Look at the CPU and Memory columns. If one program is using more than 50 percent of your CPU or more than 75 percent of your RAM, that is likely your problem. Close that program and see if the lag stops. Common culprits are web browsers with many tabs open, cloud backup software (OneDrive, Google Drive, Dropbox), video conferencing apps left running, and antivirus software doing a scan. If you see a program you do not recognize using a lot of resources, search for its name online before closing it — it might be something important.

If nothing is using an unusual amount of resources, move to the next section. Your lag is probably not from a single runaway program.

Check how full your hard drive is

A hard drive that is more than 85 percent full will slow down your entire computer, even when you are not running anything. Windows and Mac both need free space to create temporary files while you work, and when that space is gone, everything becomes sluggish.

On Windows, open File Explorer, right-click "This PC" on the left, and click "Properties". You will see a pie chart showing how much space is used and how much is free. On Mac, click the Apple menu, select "About This Mac", click "Storage", and look at the bar at the bottom. If the bar is mostly full or shows less than 20 percent free space, that is your lag.

To free up space, delete old files you no longer need, empty your Downloads folder (it often fills up without you noticing), and move large files like videos or old photos to an external hard drive or cloud storage. Uninstalling programs you do not use also helps. On Windows, go to Settings > Apps > Apps & Features, find programs you want to remove, and click Uninstall. On Mac, open Applications, drag unwanted programs to the Trash, and empty the Trash.

Scan for malware and viruses

Malware and viruses run in the background and use your processor, memory, and internet connection without your knowledge. They can make your computer lag even when you are not doing anything. The good news is that both Windows and Mac come with built-in scanning tools that are free and effective.

On Windows, open Windows Defender (search for "Windows Security" in the Start menu), click "Virus & threat protection", and click "Scan options". Select "Full scan" and click "Scan now". This will take 30 minutes to an hour, but it will check every file on your computer. On Mac, open System Preferences, click "Security & Privacy", and look at the "General" tab — if it says your Mac is find, you are probably fine. If you want a deeper check, read the free version of Malwarebytes, run a full scan, and let it remove anything it finds.

If a scan finds malware, let the tool remove it and restart your computer. Lag often disappears when ready after malware is gone.

Look at your startup programs

Many programs are set to start automatically when your computer turns on, and they stay running in the background all day. If you have too many startup programs, your computer will be slow from the moment you turn it on.

On Windows, open Task Manager (Ctrl+Shift+Esc), click the "Startup" tab, and you will see a list of programs that run when you start your computer. Right-click any program you do not need to start automatically and click "Disable". Common programs to disable are cloud storage apps (if you do not need them syncing constantly), messaging apps, and software from manufacturers that came with your computer. On Mac, go to System Preferences > General > Login Items, and remove programs from the list by selecting them and clicking the minus button.

Restart your computer and see if it feels faster. Disabling startup programs is one of the quickest ways to improve boot time and overall speed.

Check if your hard drive is failing

If you have ruled out background programs, a full hard drive, malware, and startup programs, your hard drive itself might be failing. Hard drives wear out over time, and a failing drive will cause lag, freezing, and slow file access even when nothing is running.

On Windows, open Command Prompt (search for "cmd" in the Start menu), type "wmic logicaldisk get name, size, freespace" and press Enter. This shows your disk space. If the numbers look normal, your hard drive is probably not failing. On Mac, open Disk Utility (Applications > Utilities > Disk Utility), select your drive, and click "First Aid" to check for errors.

If you see error messages or if your computer is more than five or six years old and still lagging after you have tried everything above, take your computer to a technician. They can run a diagnostic test to see if the hard drive is failing and replace it if needed. A new hard drive or solid-state drive (SSD) will make an old computer feel almost new again.

Consider upgrading your RAM or switching to an SSD

If your computer is old and you have ruled out all the software causes above, the hardware itself might not be powerful enough for what you are doing. RAM (memory) and hard drive speed are the two biggest factors in how fast your computer feels.

If you regularly use programs like video editing, photo editing, or 3D design, or if you keep 20 or more browser tabs open at once, your computer might not have enough RAM. Most computers come with 4 to 8 gigabytes of RAM. If you are doing heavy work, 16 gigabytes is better. A technician can add more RAM to most computers for $50 to $150 in parts and labor.

If your computer has a traditional hard drive (HDD) instead of a solid-state drive (SSD), replacing it with an SSD will make everything faster — opening files, starting programs, and booting up. An SSD costs $50 to $200 depending on size, and a technician can install one for $50 to $100. This is often the single best upgrade for an older computer.

Frequently Asked Questions

Why does my computer lag only when I play games or edit videos?

Those programs need a lot of RAM and graphics power. Your computer is hitting its limits. Check Task Manager or Activity Monitor while the program is running — if RAM is at 90 percent or higher, you need more RAM or a better graphics card. If CPU is at 100 percent, your processor is too slow for that task.

My computer is brand new but still lags. What should I do?

New computers sometimes come with bloatware — programs the manufacturer installed that you do not need. Open Task Manager, go to the Startup tab, and disable programs you do not recognize. Also check if Windows or Mac is doing a background update or scan. Wait 30 minutes and try again.

Does closing browser tabs really help?

Yes. Each open tab uses RAM and CPU. If you have 30 tabs open, your browser might be using 4 to 8 gigabytes of RAM. Closing tabs you are not using will free up memory when ready. If you need to keep many tabs, consider using a tab manager extension or opening them in a separate browser window.

Is it worth fixing an old computer or should I just buy a new one?

If your computer is more than seven or eight years old, the cost of repairs (new hard drive, more RAM, new battery) might add up to $300 to $500. A used computer of the same age costs $200 to $400. A new budget computer costs $400 to $600. If repairs cost more than half the price of a new computer, buying new usually makes more sense.

Will clearing my cache and temporary files speed up my computer?

It helps a little, but not much. Clearing your browser cache frees up a few hundred megabytes at most. It is worth doing if your hard drive is very full, but it is not a primary fix for lag. Focus on the bigger issues first: hard drive space, background programs, and malware.