From ActionScript Performance Wiki

Purpose

Test if it is faster to convert a number to an integer or to perform Math.round().

Code

package  
{
	import flash.display.Sprite;
 
	/**
	 * @author Nicolas Schudel
	 */
	public class Test extends Sprite 
	{
		public var a:Number;
		public var b:Number = 1.056684651654654;
		public var c:int;
 
		public function Test()
		{
			trace(new PerformanceComparison(1000000, test1, test2, test3, test4).start());
		}
 
		private function test1():void 
		{
			a = Math.round(b);
		}
 
		private function test2():void 
		{
			a = int(b);
		}
 
		private function test3():void 
		{
			a = b as int;
		}
 
		private function test4():void
		{
			c = b;
		}
	}
}

Results

Compiler version: 3.3.0.485, Player version: MAC 10.0.2.54, Operating System: Mac OS 10.5.8

Test 1

628 ms, 37% slower

Test 2

460 ms, best

Test 3

597 ms, 30% slower

Test 4

469 ms, 02% slower

Compiler version: 3.3.04852, Player version: WIN 10.0.45.2, Operating System: Windows XP

Test 1

455 ms, 46% slower

Test 2

313 ms, 01% slower

Test 3

498 ms, 60% slower

Test 4

312 ms, best

Conclusion

Type casting a number to an integer is the fastest way to round a number. Surprisingly int(n) is faster than n as int.