Sonic CD have slowdowns also on original hardware.
I think it's also the same for metal slug

If this happens on the original hardware, I don't think it's exactly a problem, so I don't believe there is a solution in the emulation.

Searching quickly I saw some users cite a hack called "Sonic CD Pal to NTSC Complete Patch", which seems to have some improvements, and can be used in conjunction with the "Sonic CD++" hack, which seems to be well evaluated. But you would need test, I don't play this game, and I didn't even know that this problem existed on the original hardware, so I can't guarantee anything.