https://wccftech.com/intel-cpu-performance-drop-with-windows-defender-heres-the-fix/

Kevin Glynn, the associate software author of PC utilities such as ThrottleStop and RealTemp for the TechPowerUP, created a new utility called "Counter Control," allowing users to monitor and log the performance counters of previous and current Intel Core CPUs. The new tool supports Intel processors launched in 2008 (Nehalem architecture CPUs) to present Alder Lake and Arc processors.

While Glynn was developing ThrottleStop, he uncovered a glitch within Microsoft's security software, Windows Defender, causing a critical impact on CPU performance. Security software of all types tends to affect performance to some degree, but this particular bug has a much higher effect on the processor while active.