Keyboard listener is killing my framerate - actionscript-3

Question

I'm working on a game that uses four simultaneous key presses. It all works fine, except that when the keys are rapidly pressed, my framerate slows down significantly (if I hammer even just one of the keys, I can halve the framerate).
I initially just assumed that there was too much going on in the method that the key press triggers, but if I take the code out of the method completely, the slowdown still occurs.
Has anyone run into this before? The keypress is one where you hold the key down, so it's repeatedly firing a method call every frame, but this is pretty standard for many uses and I've never encountered this before.
EDIT: clarification.
Structurally, there's a KEY_DOWN and KEY_UP listener attached to the stage:
stage.addEventListener(KeyboardEvent.KEY_DOWN, menuKeyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, menuKeyUp);
which then calls a method with a single switch statement in, that contains five lines like this:
case ONE: pressing1 = true; break;
There's an ENTER_FRAME event that checks to see if any of the boolean flags are true, and handles character movement. This calculation happens regardless of key presses (i.e. if you let go of the keys, gravity still has an effect). This holds 60fps easily.
The issue is literally at the exact moment the key is pressed or released, there's an almost imperceptible frame drop. Repeatedly pressing the keys causes the framedrop to get worse and worse. Holding a key down doesn't kill the framerate, apart from the single split second frame drop when the key is pressed down. After that the game carries on as normal. Letting go of the key causes another tiny hitch and then the framerate goes back to normal.
EDIT 2 - I added a framerate checker so I could see exactly what was happening to the framerate. Interestingly, I can't make it go any lower than EXACTLY 30fps even when I press keys very rapidly. Is there some sort of restriction in play here with Flash Player?

Answer 1

Turns out that the code was correct all along. Playing 60FPS content in the debug player or the standalone player causes any events (mouse and keyboard) to hitch the framerate. Viewing the same content in a browser, or exported to AIR, stops the issue entirely. The content is now running perfectly at 60fps without any slowdown.
Hope this helps someone, I was tearing my hair out!

Answer 2

Do you have multiple different listeners, or one listener that appropriately routes the keypress? I would suspect the former. The solution is to switch to the latter.
What I typically do is have one object that's responsible for listening to key presses and translating keyboard events into other, more meaningful events.
For example:
protected function handleKeyboardEvent(e:KeyboardEvent):void {
if (e.ctrlKey) {
switch (e.keyCode) {
case Keyboard.A:
eventBus.dispatchEvent(new Event(ViewEventKind.SELECT_ALL));
return;
case Keyboard.Y:
eventBus.dispatchEvent(new Event(ModelEventKind.REDO));
return;
case Keyboard.Z:
eventBus.dispatchEvent(new Event(ModelEventKind.UNDO));
return;
}
}
}