“This performance issue makes calls to VirtualProtect (among other things) unreasonably expensive on Windows platforms when Windows Defender’s Real-time Protection feature is active (the unreasonably expensive computations execute in the MsMpEng.exe process).”
This bug was opened in February 2018. A Firefox engineer finally traced it to Defender itself, and Microsoft acknowledged it. Loading YouTube five times cost Defender 16 seconds of CPU with JIT on and 6 seconds with it off. Microsoft’s antivirus spent years taxing its main browser rival and Firefox users ate the battery drain.