You can use the built-in MzScheme profiling tools; just run '(time (tl))' instead of '(tl)' to start the Arc REPL. You'll want some way to force a return from the toplevel interpreter, which may mean writing a shutdown routine for the web listener that doesn't require Ctrl-C halting the server thread, but that should be easy enough to do.
MzScheme has a better profiling facility (the errortrace collection), but using it without source information is not too helpful.
It is possible to do that (and I have a patch that does it), but it's not a quick solution.
