Why Does My Antivirus Take So Long to Scan? 6 Reasons

Your virus scan has been running for two hours and it is still going. Here is how to tell whether that is a fault or just how long it takes, and the six things that make it far slower than it should be.

Maybe it goes on for hours. Maybe it never finishes at all. Either way, it is one of the most frustrating things a computer does, because you can’t really use the machine properly while it’s happening and you have no idea how much longer it will be.

This applies whatever you are using. Windows Security, Norton, McAfee, Avast, any of them. For some people, virus scans go on forever.

Let me start with the question everybody asks first, because for a good number of you that is the whole answer.

How Long Should a Virus Scan Take?

It depends entirely on which kind of scan you have started, and most people don’t realise there is more than one.

• Quick scan. A few minutes. It checks the places malware actually hides: running programs, startup items, the main system folders and what’s running in memory. This is the one you should be running routinely. Perhaps once a week.
• Full scan. Anywhere from about thirty minutes to several hours. It opens and checks every single file on the drive. On a big drive that’s hundreds of thousands of files. Every one needs to be picked up and checked by your scanner.
• Offline scan. About fifteen minutes. Restarts the computer and scans before Windows loads.

A full scan taking two hours is not a fault. That is a full scan doing its job on a drive with a lot on it. If yours is running long and you started a full scan, there may be nothing wrong at all.

It is the same story whichever program you use. McAfee, Malwarebytes, Bitdefender, Kaspersky, Norton, Avast and Windows Security all do the same job in roughly the same time, because they are all reading the same files off the same drive. If somebody tells you their scan finishes in ten minutes, they ran a quick scan.

If your scan really is dragging, the six reasons below are what I would check, in that order.

1. You Have More Than One Antivirus Installed

This is the one I see most often, and it’s the one that causes the worst slowdowns.

Never have more than one antivirus program installed. Two of them running together will fight each other. Each one watches every file being opened, which means the second one sees the first one opening files and checks those too. Your scan crawls, and so does the rest of the computer.

It usually happens by accident. Somebody buys a laptop with a trial version already on it, then installs the one they actually wanted, and now there are two. Or a free download quietly brought one along with it.

So the first job is to look. Right click the Start button, then:

• In Windows 11, click Installed apps.
• In Windows 10, click Apps and Features.

Read down the list looking for anything that is antivirus or antimalware. Norton, McAfee, Avast, AVG, Bitdefender, Kaspersky, Malwarebytes, Webroot, and whatever your laptop maker bundled.

2. Your Computer Is Going to Sleep Mid-Scan

This is the sneaky one, and it explains the scans that seem to run all night and get nowhere.

You start a full scan, walk away, and twenty minutes later the computer decides you’ve gone and drops into sleep mode. When that happens the scan stops. It hasn’t failed and it hasn’t crashed, it’s simply paused, waiting for a computer that is asleep. You come back in the morning and find that it’s still on 34%.

Most antivirus programs can’t stop this from happening. They aren’t allowed to override your computer power settings.

So if you want to leave a scan running unattended, you have to tell Windows not to sleep first. In Windows 11 that is Settings, then System, then Power (or Power & Battery), then Screen and sleep time-outs. Set the sleep options to Never while the scan runs, and put them back afterwards.

Always run antivirus scans on a laptop with the power cord plugged in. Using battery power alone will slow everything down.

If your computer nods off far too quickly generally, that is worth fixing on its own. Stop your computer going to sleep too quickly.

3. There Is Something Wrong With the Drive

A virus scan reads every file. So anything making the drive slow to read makes the scan slow, and a drive that is genuinely struggling can stop a scan dead.

Two different things go wrong here, and which one applies depends on what kind of drive you have.

A Damaged File System

The file system is the index Windows uses to find things on the drive. It can get muddled, or corrupted, usually after a crash, a blue screen, or the power going off while the computer was busy.

When the index is wrong, every file the scanner asks for takes longer to locate. Multiply that by a few hundred thousand files and you have your slow scan. This can happen on any drive, old or new.

Check Disk fixes it. It’s built into Windows so there is nothing to download.

How to use CHKDSK (Check Disk) in Windows.

A Drive on Its Way Out

On an older mechanical hard drive, the sort with spinning platters inside, parts of the surface can go bad. When the scanner asks for a file living on a bad patch (called sectors), the drive retries over and over before giving up. That is where scans get stuck on one percentage for an hour.

On an SSD, the newer sort with no moving parts, it does not work that way. SSDs quietly move data off failing areas themselves and you rarely see the long pauses. A failing SSD is more likely to slow down generally or stop being seen at all.

Either way, if Check Disk finds real errors, or the scan keeps stalling in the same place, treat that as your computer telling you something. Back up anything you care about now, not later. There is more on checking the drive at the end of this guide.

4. It Is Scanning Your USB Drives Too

A full scan means everything. If you have an external hard drive plugged in, that gets scanned as well.

This one catches people out because the external drive is often the biggest thing attached to the computer. A backup drive can easily hold more than the computer itself, and it is slower to read, especially over an older USB connection. You can double or triple the length of a scan without realising the extra time is not even being spent on your PC.

So before a full scan, unplug external drives and memory sticks, and check there is no disc left in the CD or DVD drive.

Scan them separately afterwards if you want to. A backup drive is worth checking now and then, just not at the same time as everything else.

5. Windows Is Updating at the Same Time

Rarer than the others, but I have seen it more than once.

Windows decides to download an update while your scan is running. Both jobs want the drive at once, and both slow to a crawl. With an ordinary monthly update you might not notice. With one of the big feature updates, which are several gigabytes, you absolutely will.

The fix is simply to get updates out of the way first. Open Settings, go to Windows Update, install whatever is waiting, and restart the computer properly. Then run your scan on a machine that has nothing else to do.

Restarting matters. An update that has installed but is still waiting for a restart can leave the computer sluggish until you do.

6. Years of Junk Files Are Being Scanned

Every file on the drive gets checked, and that includes an awful lot of files nobody needs.

The big one is old Windows updates. When Windows installs a major update it keeps the previous version in a compressed folder so you can go back if the new one misbehaves. That folder can be many gigabytes, and your scanner will dutifully read every bit of it.

Disk Cleanup clears it out safely.

1. Click Start and type disk cleanup.
2. Open it, then click Clean up system files.
3. Wait for it to close and reopen. It takes a minute, because this time it is looking through the old update files as well.
4. Tick every box in the list, then click OK.

Windows 11 also has Storage Sense, in Settings under System and then Storage, which does much the same thing automatically from then on. Worth switching on once you have had a proper clear out.

The Checklist, in Order

If your scans are genuinely crawling, work through these in this order. Most people stop somewhere around step three.

1. Check whether you actually started a full scan. If so, give it a couple of hours before assuming anything is wrong.
2. Look through your installed apps and uninstall every antivirus and antimalware program, including the one you want. Then reinstall just that one.
3. Stop the computer sleeping. Set sleep to Never while the scan runs, and Power mode to Best performance.
4. Unplug external drives and memory sticks, and take any disc out of the CD or DVD drive.
5. Install any waiting Windows updates and restart.
6. Run Disk Cleanup, including Clean up system files.
7. Run Check Disk.
8. Now run a full scan with Windows Security on its own. If that finishes in a sensible time, put your preferred antivirus back and it should behave.

Do them one at a time and try a scan in between where you can. Otherwise you fix it and never find out which one was the problem, which is no help at all next time.

If what you actually want is a second opinion rather than a faster scan, that is a different job. Scanning for malware your antivirus missed covers running Malwarebytes alongside what you already have, which is the one safe exception to the never-install-two rule.

If It Still Will Not Finish

You have been through everything and a Windows Security full scan still stalls. Two things left to try, and they point in different directions.

Run an Offline Scan

Some malware interferes with the scanner looking for it. The way round that is to scan before Windows has finished starting up, when the nasty thing has not had a chance to load.

Windows has this built in and hardly anybody knows about it.

1. Open Windows Security.
2. Click Virus & threat protection.
3. Click Scan options.
4. Select Microsoft Defender Offline scan, then click Scan now.

Save anything you have open first, because it restarts the computer within about a minute. The scan itself takes roughly fifteen minutes, on a screen that looks nothing like Windows, and then it restarts again by itself. That is all normal.

Fifteen minutes for the thorough one is worth knowing about when the ordinary scan has been going for four hours.

Stuck on the Same Percentage

A scan sitting on 27%, or 91%, or stuck at 100% and never closing, is its own particular kind of maddening. It is nearly always one of three things.

• One enormous file. A big backup file, a disc image or a virtual machine can take a very long time on its own. The percentage has not frozen, it is just not moving.
• A part of the drive it cannot read. The scanner asks, the drive retries, and nothing happens for a long time. Run Check Disk.
• It finished and did not notice. Stuck on 100% usually means the scan is done and the program has not tidied up after itself. Close it, reopen it, and look at the scan history.

Give it an hour on the same number before you decide it is stuck. Then stop the scan, which is perfectly safe, and work down the checklist above.

Check the Drive Itself

If the offline scan finishes fine but ordinary scans still stall, look harder at the drive. A drive that is slowly failing will do exactly this.

Every drive manufacturer has a free tool for checking their own drives, and those are the ones I would use: Seagate’s SeaTools, Western Digital’s Dashboard, Samsung Magician, Crucial Storage Executive. Find out which make your drive is and use theirs.

And before you run any of them, back up anything you would be upset to lose. If the drive really is failing, the worst possible time to discover it is halfway through testing it.

Frequently Asked Questions

Summary

Start by asking whether the scan is actually too slow. A full scan reads every file on the drive and taking one to three hours over it is simply what that job costs. If that is what you have got, there is nothing to fix.

If it really is dragging, it is nearly always one of six things. A second antivirus fighting the first, the computer dropping off to sleep partway through, a drive with errors on it, external drives being scanned as well, a Windows update competing for the same drive, or years of old update files being read for no reason.

Work down the checklist in order and most people never reach the bottom. If you do reach the bottom and it is still stalling, try the offline scan, and take seriously the possibility that the drive is telling you something.

At Home Computer Guides

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You can find this and other related step by step guides in – At Home Computer Windows 11 Guides