Flash on Linux doesn't seem to use hardware acceleration for video, so it is really processor intensive when resizing to full screen. I think this is to be fixed for the next version of Flash, but I'm not sure.
If I want full screen flash video I use mplayer instead. Mplayer can play .flv files, but it doesn't work on the included version, so if you want to use it for that you need to uninstall the included version, add the livna repository to yum and reinstall mplayer with the version from the livnea repository. To play the videos I typically just pause the Flash player and then run mplayer /tmp/Flashxxxxxx where the x's are a random string of characters. This only works where the video is saved as a temporary file, but it certainly works for YouTube.