Ok, I tried that. Running the fib code in a mzscheme REPL is slower than in an Arc REPL. But, if once in Arc REPL you type :a (and get to the mzscheme REPL with Arc functions loaded), the same Scheme code is then a little faster than the Arc counterpart. That means there is something in ac.scm that speeds up mzscheme computations. Maybe the fact that it's embedded in a module ? I really don't know, but at least this is not an aberration anymore.
