I believe I have dsevocired why the forward loops are faster in AS3: nJIT.It sees that you're comparing against a constant, and inlines it, making the effective bytecode:getlocalpushintifge jumpWhile the bytecode of the backwards loop is effectively (there may be more to it, I've noticed that adding gt 0 can speed it up):getlocaldupincrementsetlocaliffalse jumpAnd the forwards loop is slower in because you used var instead of const making the equive AS3 bytecode:getlocalgetlocalifgr jumpget/set local are slow operations, if you use a var in AS3 you should also see the slowdown as in JS.The slowdown/speedup in AS2 however is confusing. I can't come up with a remotely valid reason for this, or even how to do it on the player's end.