Update: I've been using the JIT version for the last day (my normal preference is for the 3m version), and it's really nice! I see speedup near a factor of 2 for much of my code (which is pretty loop-heavy, so maybe that helps get the extra speedups), and the whole experience of using DrScheme feels much more snappy!
01 February 2006
According to this message to the PLT Scheme mailing list, PLT Scheme just got a JIT compiler. It only works on the non-3m (i.e. Boehm conservative collector) version of PLT Scheme, but it seems to provide some speedups. Subsequent messages suggest that they're working on a "proper" JIT compiler targeting LLVM which should provide even more speed. Very nice!