The Problem:
When you enter the main menu (v. 4.3.x) you get only 2 fps max with a gpu usage of 100% and it is almost impossible to click anything.
Verifying the game files, or reinstalling the game did not help at all.
It's also a very strange load on the gpu, I haven't encountered in any other game before.

Specs:
In my case a 5800X3D with 64GB DDR4 3800 paired with a RTX 4090 and a ultra widescreen LG with 4800x2000 (24:10) resolution. ...but as it is fixed for me I don't have a screenshot showing the 2 fps in the menu unfortunately. :/ :D

Fast-Fix (who knows?)
If you are able to switch to borderless in the graphics settings - despite the 2 fps "limit" - go for it and try it. I was so sick of the annoying cursor.

1. Righ click on SQUAD and select 'properties'.
1.1 Go to BETAS and choose: 'performance_hotfix - branch...'
1.2. Steam starts to download the whole game (the new branch)

2. start the new branch
2.1 Yes, suddenly it is super smooth, but with DX11.
2.2 Go to settings and under window mode just select 'borderless'
2.3 Apply settings and quit game

3. Righ click on SQUAD and select 'properties'.
3.1 Go to BETA and de-select the: 'performance_hotfix - branch...'
3.2 Now steam downloads the whole game, again.
3.3 Start the normal version of SQUAD.
3.4 Yes, suddenly it is super smooth, now also with DX12.

4. Enjoy the SQUAD menu full frames experience.
4.1 Yes, wtf.

PS: Hope ya got a fast internet connection.

And finally as it was before.