I have Rebar enabled. Forza Horizon 4 is one of the games which benefits heavily from Rebar enabled. But it's maybe behaving differently on Linux.
I will try.
Note that I'm not the only one who is having these issues, there are people in protondb saying the same, even in 2023. This is why I avoided Steam version & played Microsoft Store version back when I used Windows.
it's that Tekken7 and Horizon Zero Dawn, stuttered to hell on my R6600/R3300x PC while it worked well on the steam deck, and all I did was shut off the rebar and everything turned butter smooth on bazzite-deck and CachyOS