How fast is forth compared to C these days?
People say that forth isn't very optimisable for our register machines, but I reckon that you can get pretty good results with some clever stack analysis. It's actually possible to determine arity statically if you don't have multiple-arity words, which are very rare. That allows you to pass arguments by register.
Anyway, I'm not even close to an expert so don't take what I said as facts.