Fixes are in the works, but are not out yet.
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If you are using an AMD based CPU and you plan on upgrading to Windows 11, you may want to hold off on that for a little while longer. Hell, I still say that you avoid Windows 11 regardless of your CPU for a good year or more if you can help it as it's just not in a good state right now.

But if you're moving forward with Windows 11 despite this warning, you should be aware that you may experience reduced system performance by about 15%. This performance dip was officially stated by AMD through a new support page. It has also been acknowledged by Microsoft.

AMD says that at least one of the issues concerns how L3 cache latency is measured. In Windows 11, latency of the L3 cache can be nearly triple what it should be, which reduces performance for applications that rely on speedy memory and cache latency. This most certainly includes gaming performance.

The other issue noted by AMD concerns the "preferred core" feature not working as it should be. Preferred core is supposed to assign software to use the fastest core on the CPU. In Windows 11, it isn't doing that as it should be.

Known Performance Changes Impact Resolution
Measured and functional L3 cache latency may increase by ~3X.
  • Applications sensitive to memory subsystem access time may be impacted.
  • Expected performance impact of 3-5% in affected applications, 10-15% outliers possible in games commonly used for eSports.
  • A Windows update is in development to address this issue with expected availability in October of 2021.
UEFI CPPC2 (“preferred core”) may not preferentially schedule threads on a processor’s fastest core.
  • Applications sensitive to the performance of one or a few CPU threads may exhibit reduced performance.
  • Performance impact may be more detectable in >8-core processors above 65W TDP.
  • A software update is in development to address this issue with expected availability in October of 2021.

AMD says that over 100 of their Windows 11 supported CPUs could be impacted by this. This also includes Ryzen processors.

AMD says that there will be a software update and a Windows update released later this month that should fix these performance issues. As with all things, don't be too surprised if these patches are delayed out of this month's planned release.

In related news, PC Gamer noted that gaming performance on all computers could tank by as much as 25% in Windows 11 compared to Windows 10. Apparently the main culprit seems to lie with the fact that new machines with Windows 11 pre-installed may also have Virtualization-Based Security (VBS) enabled by default. While this is something that can be toggled off, those who aren't aware of it may not realize they aren't getting the performance they should be with their new system.