[cjbos]

This is how flash player now works, with the latest release it drops the frame rate of the flash movie to something like 2-3 frames per second if the movie is not visible in the browser.

Flash player is also single threaded so executes code on a single cpu core, but it does rendering using multiple cores. If you are seeing a single core spike to 100% usage then it is a sign of badly coded actionscript (onFrameEnter event handlers are very popular with designers who cut and paste as2 code), if you are seeing multiple cores at around 100% then it is a rendering issue, i.e. trying to redraw the entire stage every frame and/or a very high frame rate (> 30 fps).

I'm not sure if code attached to the frame in the Flash IDE would also cause multiple cores to spike, or if this also executes in the single threaded cycle. But again, having actionscript inside the .fla instead of separate .as files is a sign of a poorly designed/developed flash application.