Bug in Microsoft Defender antivirus slows down computers with Intel processors

Developer Kevin Glynn while working on his ThrottleStop utility, which prevents CPU throttling, discovered non-standard behavior of the standard Microsoft Defender antivirus (formerly Windows Defender). For no apparent reason, the application starts switching performance counters to the active position, using more processor resources.

For the sake of context: under such an additional load, the Core i9-10850K loses about 1,000 points (6%) in the Cinebench benchmark. A similar drawdown is observed for Intel processors from generations 8 to 11 on Windows 10 and Windows 11 both in desktop and laptops. 12th generation AMD and Intel Core processors are not expected to be affected by this bug.

In this case, the user will not even understand what is happening: the computer will simply slow down, and you can only notice something is wrong in the process manager. Glynn notes that the problem is software, and resetting the counters manually allows you to instantly silence Defender without compromising system monitoring.

In theory, this can be solved disabling the antivirus, but this puts the computer's security at risk. Another method is to install the utility Counter Control, which Glynn wrote after discovering the bug. The application monitors Defender activity, and when it starts to use all seven performance counters, it resets them to their original state so that the PC can work normally.

So far, Microsoft has not responded to this problem, but it is likely that the bug will be fixed in one of the next updates.