Turn your settings all the way down, turn off Vsync and use GTAO Land Races to hit stunts for max framerate. FPS impacts the BMX more than any other vehicle. 120+fps BMX is a completely different game compared to 60fps BMX. It's wild. You probably already know this but I still wanna bring it up for others interested.

Also, I really suggest getting a controller. The very small movements are massive and IMO you need linear triggers for the front brake. Pressing it in 20-30% is much more effective for wallclimbing/boosting. The front brake has a ton of applications I could spend hours listing, it's probably the most important mechanic for BMX stunting imo.