It's a bit of the fault of "portability"... like MAME. It's slow because it's unoptimized, so it allows portability to other platforms. SIDPlay 2 runs fine on my 866 system, even if I open two instances of it and run a very intensive tune on both... But you have to admit the SID chip is a very complex beast, specially on the filter side. Since teh filters are analog they haev to be simuate, and I think they get problems there. Even the old SIDPlay would struggle with my Pentium 133...
Running on Windows also helps it to be cack
I think most of teh conversions of C64 tunes to teh Amiga were poop, because the "digitalization" busted them. Samples do NOT mean better music. The "reworking" means they replace teh synthetized sounds with samples, and it loses teh "feel" in the process. It is like if you were to remake a Piano tune, and you use a guitar sample instead. it will be crap. Why? Because the piece was intended for play on a piano. Same goes with teh SID. The original tunes were intended to be played on the SID chip, not with crap guitar samples.
Ask martin Galway why he never remade any of his SID tunes, and have a listen at most of these now popular SID remake MP3s... Those that do not use the original SID sounds have lost its essence.