So the main thing to keep in mind is that a for..in loop is now roughly 28 times slower than a standard integer loop - in AS2 the impact wasn't that big. Also don't overload the loop header, but put everything in the loop body instead, for example: // slower var k:int = 1000; for (var j:int = k - 1, i:int = 0; i < k; j = i, i++) { } // faster var k:int = 1000; var j:int = k - 1; for (var i:int = 0