I will be updating this post with my findings as I go. Intel 8600k at 4.8 Ghz, 48Gb Ram at 3200 Mhz, RTX 3080 10gb.
Using the Cessna Skyhawk 172 at JFK on runway 31L. Time of the day 1:00 pm. Few Clouds. 4K resolution, MP off, AI traffic off, LOD 300, All settings ultra and photogrammetry on.
With all WUs I was getting 19 FPS on the runway. Main thread limited.
Restarting the sim after each change:
Removing all WUs I got 43 FPS.
Adding USA also 43 FPS so no performance issues there.
USA + Japan 35 FPS. -8
USA + UK 35 FPS. -8
USA + France 34 FPS. -9
USA + Nordics 41 FPS. -2
USA + Germany 35 FPS. -8
Just for the fun of it and to rule out that the issue may be related to the number of WUs installed (at least 2) I tried removing USA and leaving only Germany on. With only Germany I get 35 FPS the same as with USA installed so I can confirm USA has no performance issues.
This issue is easily reproducible in my system and hopefully the developers can figure out what the root cause is. Until then I will use this sim with USA + Nordics. It’s a shame because I was actually able to enjoy France and the UK with acceptable performance in SU6. Oh and please also look into the Track IR issue 
Edit 12/1/2021
After further testing I found out that uninstalling the whole WUs package isn’t necessary to get the FPS improvements. Only the 3d photogrammetry cities parts need to be deleted. Airports, points of interest, procedural buildings and bushtrips have no effect in performance.
Here is a screenshot of what I have deleted and I have great performance on the ground in the same testing scenario I described:
Edit 12/16/2021
Thanks to @propilot23k suggestion I also tested removing the GOTY 3D photogrammetry objects. In my previous testing when I mentioned USA it actually meant USA + GOTY. Removing GOTY 3d photogrammetry objects gave me a 7 FPS boost on the ground.
So my current content looks like this for maximum performance in the USA:
Edit 1/31/2022
Australia 3D photogrammetry cities gives me a 10fps penalty when in the USA JFK on the ground so I will keep it off also unless I decide to fly around it.
Adding all the 3D photogrammetry cities now brings my fps on the ground from 48 all the way down to 19 in the USA JFK on the ground. Seems like the more WU 3D photogrammetry cities I install the worse the performance gets. This behavior was expected so I will keep the USA and Nordics fully installed as usual and all the other 3D Photogrammetry cities uninstalled.