Are you kidding? The emulator is still slow (compared with a real DC) even on a P4 1.5Ghz. The authors want to optimize it so that it can run fast on slower machines but it will take some time (and hard work). You'll probably have to wait for the emulator to be more stable and compatible and after that (after some months/years) it may run relatively fast on a 700Mhz processor
I agree, support for that speed may become available eventually in Chankast, but in my opinion it is still a ways off. Best to just buy a Dreamcast if you really want to play one.