Key.isDown/onKeyDown slowing the movie?

I’m trying to make a fairly ambitious game that’s probably a bit hard on the CPU, and no matter how I implement the controls it slows down the movie after a split second. What I’m seeing is that holding down a key… ANY key… will cause the framerate to drop after a split second. But if you tap a different key and still hold the first one, the framerate increases again and the function of the original key is not interrupted. At first I thought the onKeyDown event I was using to capture keystrokes was trying to run too fast, like when you’re typing and you hold down the letter F and it starts going “FFFFFFFFFFFFFFFF” after a second, but even when I changed it to Key.isDown it still does the same… and keys that I’m not even trying to register are causing the same slowdown.

So… does anyone know a work-around for this?