I've noticed this problem with the Formula NA 1999, 2012, and 2018 (Not 2021, that one, as well as VRC's newer cars, work fine) on aa_ims and various other tracks (I am also having this problem with URD's Formula Lights). I usually race with 19 AI on 82+-1.5 strength and these cars grant me about 19FPS on aa_ims. I am running on a fairly decent rig (GTX 1660, i7-9700F clocked at 3GHz). I did see other people racing with the NA 1999 on IMS with no problems. Not sure why this is happening.
I tried disabling all apps, using only default apps, lowering graphics settings, disabling CSP (Which did work, but it removed the skybox), removing the extension folder from the track, no dice.