This might be a little late, but worth keeping an eye on. As we know since Flashplayer 10.1, the player is throttled in situations. What does that mean is the FlashPlayer will run the EnterFrame and Timer based events at much lower rates, when the player is not visible. Much lower means, as low as 2 FPS (2 frames per second) and AS2 Intervals as well as AS3 timers will low down to 2 events per second!!
This throttling is best for mobile and other devices which can run FlashPlayer now. Throttling means saving of energy and increasing battery life of the device. So it seems its the best thing happened to Flashplayer, while we are making content for the devices.
The problem arises when we create stuffs for desktops or laptops, where FlashPlayer was running since a long time, conventionally. And we still think we can go on developing our applications and games with the same mind set as we used to do till now. While developing games, the most used event is ENTER_FRAME and when throttling happens, all of the logic of the application/game goes wrong!! While the logic of the game still holds good if the player is active all the time but once the player is invisible, the player starts to throttle and frame-rates and timer events start to fall. This will happen even our game is running in a browser tab and we are in another tab. This can happen if we scroll down a page and the game is on the top portion of the page! The tricky part is different browsers behave differently and there is no clear idea about when the player starts throttling. So there is no fix solution for the same too.