Performance for Wreckfest has typically been quite poor (and still is under the latest stable driver). Such that under Windows for Ultra quality I get 90-100fps, while in Proton it drops to 15-25fps.
However, the last few beta drivers gave a decent boost I think starting with the Doom: Eternal tweaks which bumped performance up to 35fps with all cars on screen (this is usually 80fps in Windows) and averaging 60-70fps through a race.
Whatever has been getting tweaked, keep doing it! Might get full performance soon.
PS: The game generally runs absolutely perfect, except for performance.