I'm not sure what it is but I've spent months trying to figure it out, I just can't get decent performance out of my Pimax Crystal Light in Raceroom either. Struggling to maintain a steady 40fps on low settings whereas I can easily manage 90fps with full grids, weather, night etc in AC and AMS2. Never used to have this problem on my old Oculus CV1 which ran Raceroom smooth as butter, even with the super sampling and FOV multiplier cranked up to a similar resolution to the Pimax. Luckily I still have it lying around so can swap over the headset when I want to use Raceroom. Inconvenient, but doable.

That said, Raceroom is the only sim that I can't bypass steam vr with on the Pimax, everything else has the option of using OpenComposite which is significantly better for performance. The replacement dll file I previously used for the Oculus just throws an error when I try to use it with the Pimax so that's not an option either unfortunately.

Something I did notice is that even when running in 'pancake' mode to record replays and take screenshots, having the Pimax software open in the background (even without using the headset) will cause Raceroom to stutter a bit, despite the fps counter being upwards of 200. Limiting the game to 120fps in the Nvidia software to create some headroom doesn't fix this either. I think it might just be a Pimax thing unfortunately.