A while ago MAME also started cleaning up it's code base to remove "hacks" or "cheats" that helped certain games to run at all, or to run better. The reasoning for this is MAME has always been about preservation and purity. It would be inaccurate to emulate the game as it did not act on its original hardware.
Outside of that, most performance issues with games come down to different PC's and configs; not much can be done in that case.