Asp Spectrum 128k Emulator runs really well on 040/25. May need to tweak settings on 030. MMU required for fast 128k emulation.
Magic64 ran really well on 040/25 too. I could run 1/1 frames with sound. I just had to dissable 1541 drive emulation while playing game as that caused a noticable performance hit. I cant rememember if that was with rtg screen though. Also performance was likely all game dependant as well.