I read a really good "intro" level article by Ted Patrick on performance in the flash player. I'm sure we've all read this stuff before, but it was a really good reminder for me, and the article is short.
Here are the best parts:
"The Flash Player uses a logical unit of work called a frame which consists of 2 phases which occur one after another in a loop:
1. Process ActionScript
2. Render Graphics
3. GOTO 1
After each of these phases is a buffer to handle delays ( say you render to much or loop over a ton of data ). When both of these phases have completed we call that a frame. Each SWF file you create has a framerate (frames/second) which represents the fastest rate that the Flash Player will loop over these phases. When Flash Player encounters delays in processing either phase it will elongate the time to process the frame and thus your framerate will slow but will never exceed the framerate set in the SWF file."
"Flex was built to handle application behavior in a minimalist fashion if used properly. I continue to see developers execute to much ActionScript on a single event and not think in terms of processing data across frames. Using events properly is a great place to start. Make sure you know how long and how often methods are being called from events."