Deleting the hole foliage does not fix the issue. Also the telemetry plugin does not fix anything at my machine.
I was able to get almost same performance like I got with 5.3 by doing:
UnrealEditor.exe path_to\ProjectName.uproject -run=DerivedDataCache -fill to fix the incredible stuttering each few msecs. Seems like the automatic-cachinge feature is bugged and/or it’s not generating required files after upgrade anymore.
r.Nanite 0 to get about double the FPS. In 5.3 it’s same scene and runs much better (even with r.Nanite 1 there’s no difference). As far I’m aware none of the meshes uses nanite. Nanite is ok as long as it does not get into my way. But even disabling Nanite at all meshes in the map does not fix the issue. Neither removing the foliage. The only difference is 5.3 (is ok) vs 5.4 (huge performance loss). If Nanite makes the performance extreme worse its a big no. It’s better to make proper retopo even in case if you might do some collisions anyway.