It's an emulator collection but their goal is perfect compatibility over any effort to optimize performance. As an example, with MAME (the arcade counterpart of MESS), when trying to emulate analogue circuits, they'd rather strive for near perfect emulation via very expensive maths operations rather than just use a first or second order approximation.

So a significant portion of what makes Dolphin so good would be rejected from MESS on principle. That isn't an inherently bad thing because MESS/MAME isn't really intending to be efficient & "fun" but rather is an archival project designed to allow perfect reproduction of the works indefinitely into the future.