Tested using core from 25.06.05. The title screen for Rockman X2 on MiSTer has lag frames when X shoots his buster shot, but doesn't on a real SNES and original cartridge. Here's a video comparison: https://www.youtube.com/watch?v=KgS3mVSKoK4
I know this is minor, but since there isn't much happening on the title screen, the presence of lag might point to some Cx4-related inaccuracy.