I tried Metal Slug X (on my p4 pc) on both FBA and MAME, and then I loaded both of them side-by-side and watch the attract sequences. The speed and framerate is exactly the same for both play and side-by-site tests.
You could try the Enhanced Blitter with default settings, and/or try Video -> Blitter Options -> "Disable All Effects". Also make sure that under "Misc -> Options -> Use Assembly MC68000 core" is OFF (unchecked).
best regards,
- dink