// fastest way to iterate through a loop
// - set var to :int
// - count down from desired loop count to zero
// - on each count we test the Boolean value of j (j<1 makes it false)
// - instead of counting down with j-- we use j += -1 because adding negative
//   numbers is faster than subtracting
//

for(var j:int=10; j; j += -1){}

http://www.calypso88.com/?p=174

Fast loop iteration

ActionScript 3