http://www.streaminglearningcenter.com/articles/flash-player-cpu-hog-or-hot-tamale-it-depends-
.html
Overall Conclusions:
When it comes to efficient video playback, the ability to access hardware
acceleration is the single most
important factor in the overall CPU load. On Windows, where Flash can access
hardware acceleration, the
CPU requirements drop to negligible levels. It seems reasonable to assume
that if the Flash Player could
access GPU-based hardware acceleration on the Mac (or iPod/iPho