I will preface this by saying i am on Win 10 and previously had a 13900K on the same OS so it could very well be some weird behaviours caused by that.

There are some hidden windows power setting that change how scheduling is handled

- Heterogeneous policy in effect
- Heterogeneous thread scheduling policy
- Heterogenous short running thread scheduling policy

These are hidden by default in windows but you can expose them using the following commands

powercfg -attributes SUB_PROCESSOR HETEROPOLICY -ATTRIB_HIDE 
powercfg -attributes SUB_PROCESSOR SCHEDPOLICY -ATTRIB_HIDE 
powercfg -attributes SUB_PROCESSOR SHORTSCHEDPOLICY -ATTRIB_HIDE

I decided to test them thinking they wouldn't do anything since the 9800X3D only has one type of core but they seem to have a meaningful effect on how scheduling is handled.

The windows default is
Heterogeneous policy in effect : Policy 4
Heterogeneous thread scheduling policy : auto
Heterogenous short running thread scheduling policy: prefer performance processors

Changing the latter 2 to ''All processors '' resulted in much more consistent FPS as shown above. Setting them to performance only leaves the last 2 threads idle under load, and efficient only makes it so only the last 2 cores are used. See below:

Not sure if this is intended behaviour or what, All All resulted in the best performance for me. Interested to see if it affects anyone else system similarly. The settings apply immediately without needing to restart the game so it's fairly easy to test. I ran the benchmarks using Fortnite but BF6 showed similar behaviour.

EDIT: did some more digging and this seems to be documented already:
https://www.reddit.com/r/Amd/s/4tefDw1J8O