The speed of scummvm on the amiga is fantastic compared with other official ports. So there was a lot optimised. You can even play it on a 020 (my 600). For instance the atari port doesn't even play very well on a 060. So the speed is awesome.
You can't compare this with shapeshifter which works in a complete different way.
If you want full speed with music and everthing you need to get a 060.