Super Double Dragon displays 6 enemy sprites on screen at once - two per character. The SNES has four modes for sprite sizes, and all sprites have to be the same size. in Super Double Dragon, to save VRAM, all sprites are 16x16 big instead of, say, 32x32 or 64x64. This lets them make rectangular sprites for tall skinny characters without wasting blank pixels, but the downside is that the SNES now has to process 6 objects on screen at once, plus two for the player. The SNES lacks registers to do this. The more objects to process, the longer the calculation time. By contrast, Final Fight on the SNES needed to limit itself infamously to just two enemies on screen at a time, both 32x32 big, to cut down on the number of objects on screen. SNES Final Fight has less than half the objects on screen as Super Double Dragon.
Again, this is why the Sega CD version of Final Fight could, by comparison, throw 6 enemies on screen at once. Not only does the 68000 have more registers to work with, the Sega Genesis VDP is also more flexible in terms of sprite size -- every sprite on screen can be any arbitrary power of two size. This means you can create a rectangular sprite with just one sprite, instead of needing to layer two onto each other. So the Genesis can put more enemies on screen at once, because each enemy occupies less VRAM and takes less objects to make up, and it has more registers to hold them.
It's truthfully not surprising at all that a game like Super Double Dragon doesn't run at 60 fps.
