A bug, discovered by TechPowerUp associate software author Kevin Glynn, causes Windows Defender to “randomly start using all seven hardware performance counters provided by Intel Core processors.” A utility Glynn created that monitors and logs performance counters on Intel Core CPUs since 2008 found that the strange behavior results in significantly reduced performance.
Bogged down by Defender hogging CPU time, a Core i9-10850K running at 5GHz loses 1,000 Cinebench points, which is about a 6% drop from the norm. Owners with Intel Core 8th, 9th, 10th, and 11th Gen processors, on both desktops and laptops, have noted similar performance hits.
As TechPowerUp notes, the underlying problem is that Windows Defender will randomly start using all seven hardware performance counters, including three fixed-function ones. Each counter can be programmed to a different privilege mode and is shared among multiple programs. For whatever reason, Defender is randomly changing the privilege level of the counters, creating a conflict with the programs trying to use them at a different level. It can happen at boot and sporadically thereafter.
To be clear, this is not an issue with Intel processors, because manually overriding the counters and resetting them returns a system to normal performance. There is no way to prevent Windows Defender from harassing your Intel processor unless you download third-party software.
Another way of overcoming this bug is by downloading software created by Glynn called Counter Control, which identifies when Defender starts using all seven performance counters and “resets” them to their appropriate state.
A more permanent solution is to download TechPowerUp’s ThrottleStop v9.5 software and enable a feature called “Windows Defender Boost” in “Options.” This setting activates a programmable timer that Defender sees and reacts to by ceasing to use all the counters.
Source: Windows Defender Bug Could Be Slowing Down Your PC