Proprietary GPU drivers have specific optimizations targeting popular games and game engines, and it's one of the planes of competition between GPU vendors. For example, Intel has a hard time with its Arc GPUs, because they didn't have 2 decades to optimize the drivers for all the games people want to play.

But AFAIU with mesa, most optimizations are done on NIR level. Does that mean all mesa drivers share the same set of game-specific optimizations?