I don't profess to be an expert in this area. I just know enough to have an only slightly better than vague idea of what goes on. The way a Tool Box function is invoked is based on the 68k processor architecture and cannot be trivially mapped onto the PPC architecture without the 68k interrupt scheme which requires an emulator. Because the MacOS relies on the 68k interrupt scheme for Tool Box functions, there is no easy mapping beyond using the emulator. Because of the interrupt scheme and the resulting non-native interrupt handler on PPCs, anything that relies on interrupts ends up executing 68k code at some point.
Time scheduled operations rely on interrupts.
The Tool Box calling convention relies on interrupts. All previous interrupt services relied on 68k style interrupts and that all is baggage can't be dropped without also dropping everything else. Apple can keep the same API, but if everything is going to be native, it will need to change the method by which these API functions are called. The problem with snappiness isn't necessarily even 68K code, by the way. Well, that's true, until the machine hits a load and the UI gets paged out.