Comment by mishoo

I can see what happens there in JSCL.. I took a look at

    (disassemble (lambda () (loop for i below (expt 10 8) count t)))

Seems TAGBODY is compiled into a while(true) which sets an exception handler on each iteration. (and then, LOOP macroexpands into a TAGBODY). I don't think there's a better way to do it, BTW — JS is simply not a friendly compilation target. But `try` on each iteration is quite costly, that's most probably the bottleneck here.