Forza Horizon low FPS and low GPU usage

I'm getting around 20 FPS on FH4 @ 3840x2160 using a 7900xtx on a system with a 3900x and 64gb ram., it's only using c20% of the GPU according to the benchmark. The same titl;e works fine on the steam deck using no specified proton version, but suffers form the above symptoms using Bazzite, has anyone got any ideas as to what could be causing it? it's pretty much the only game I've come across that hasn't worked flawlessly!