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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →To check whether antivirus software is slowing your Windows PC, open Task Manager while the slowdown is happening and compare its CPU, memory and disk activity with the scan activity shown in Windows Security. A spike that lines up with a scan is a clue, not proof: other processes, low storage, startup apps or older hardware can also make a PC slow.
Check resource use while the PC is slow
- Save your work, then press Ctrl+Shift+Esc to open Task Manager while you can feel the slowdown.
- On Processes, look at CPU, memory and disk use. Note which process is using resources and which resource is busiest. Security software process names differ by provider, so identify the displayed name rather than guessing.
- Select Performance to see overall CPU, memory and disk activity. Compare what you see during a slow period with a time when the PC feels normal. Microsoft recommends reviewing processes and the Performance and Startup apps views when troubleshooting Windows performance: Tips to improve PC performance in Windows.
There is no universal CPU or memory percentage that proves antivirus software is responsible. A busy process matters most when its activity repeatedly matches the slowdown.
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Check whether a scan coincides with the slowdown
Open Windows Security > Virus & threat protection. The page shows scan options and information about the last scan, including when it ran and how long it took. Compare those details with the times you noticed the PC slow down. Windows Security labels can vary by Windows version and update.
Microsoft says Defender checks files and processes that are opened or downloaded, and that a scan can use processor and memory resources. If a scan seems to be contributing to the slowdown, close apps you do not need and run or schedule the scan for a time when the PC is idle. See Microsoft’s guidance on troubleshooting malware detection and removal and Virus and Threat Protection in Windows Security.
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Interpret the timing carefully
A scan and slowdown happening together supports a possible connection; it does not establish that the scan caused the problem. Repeat the observations across more than one slow period, noting the process and resource involved, and compare scan periods with times when no scan is running. If another process is consistently using the CPU, memory or disk, the antivirus may not be the main source of the slowdown.
Do not turn off real-time protection or add exclusions as a performance test. Exclusions stop real-time checks on the selected files or processes and can leave the device or data vulnerable.
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Check for overlapping antivirus products
Look through installed apps and Windows Security to see which security products are present and whether more than one antimalware product may be active. Microsoft warns that running multiple antivirus or antispyware programs at the same time can reduce performance, make a PC unstable or cause unexpected restarts. Microsoft also says Defender automatically turns off when another company’s antimalware app is installed. Its guidance covers Windows 10 and Windows 8.1: Protect my PC from viruses.
If you remove an unwanted security app, make sure a protection product remains enabled. Do not leave the PC unprotected as a troubleshooting shortcut.
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Rule out other common causes
Antivirus activity is only one possible explanation. Microsoft’s Windows performance guidance also points to limited storage, too many startup apps, background apps, outdated software and older hardware. Check these alongside Task Manager rather than assuming one cause explains every slowdown:
- In Task Manager, review Startup apps for apps that launch automatically and are not needed at startup.
- Check available disk space; a drive with little free space can contribute to poor performance.
- Look for apps doing substantial background work or updates during the slow period.
- Consider whether the PC’s hardware is old or limited for the work you are asking it to do.
Microsoft’s Windows performance tips cover these possible contributors. More than one factor can be involved.
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Use Protection History for recent Defender activity
If Microsoft Defender is in use, open Windows Security > Protection History to review actions Defender has taken and related alerts. Microsoft says Protection History retains events for two weeks, so older events may no longer appear. Details are in Protection History in Windows Security.
Windows version and support
The cited Windows Security and performance guidance applies to Windows 10 and Windows 11, though labels can differ across builds. Microsoft states that Windows 10 support ended on October 14, 2025. If you still use Windows 10, check your support situation rather than assuming it continues to receive free security fixes.
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