Yeah, I remember DreamSNES on the Dreamcast ran like a dog, so not surprised that a lot of assembler is needed to make it work on platforms of that era.
The original bsnes back in the day (before higan and the modern bsnes) required quite a high end machine to run playably in accuracy mode so yeah, this all tracks.
The 90 MHZ CPU for the dreamcast is not that powerful. A Pentium MMX would barely run ZSNES with a frameskip value of 2, 8000 hz audio and the lowest supported res.