Things are a bit rosier with Adobe's pre-release of Flash Player 10.1. The Windows version has significant improvements, namely support for hardware-accelerated video decoding of h.264 video, but the Mac and Linux versions do not include support for this feature. As usual, Adobe blames Apple for "not being open enough" -- "Mac OS X does not expose access to the required APIs" according to Adobe -- but even without hardware-accelerated video decoding, this pre-release build of Flash boasts significant CPU load improvements over its predecessor.
Anandtech tested the performance improvement of the pre-release version of Flash Player with the following results:
I took the same Office clip I'd been using for all of the other tests and ran it on my Mac Pro at full screen (2560 x 1600). Using Activity Monitor I looked at the CPU utilization of the Flash Player plug-in. I compared both versions of Flash and saw a significant drop in CPU utilization:
Flash 10.0.32.18: 450%
Flash 10.1.51.45: 190%
Going from roughly 450% down to 190% (or a bit over 10% of total CPU utilization across 16 threads) made full-screen Hulu playable on my machine. In the past I always had to run it in a smaller window, but thanks to Flash 10.1 I don't have to any longer.