Or you can go with Gambit, which does superb job here and produces optimized code. Last releases added backends for Java, PHP, javascript and perl I think.

I'd reccomed guile or cyclone of you really need native threads: because of Cyclone, the potential is there, but it would be very, very hard.

Chicken does have fork, concurrent native callbacks and mpi, among other stuff, so you can do parallel stuff. It's just more expensive.