Adobe claims there's a mismatch between QTKit and Flash Player's rendering model, so using QTKit would either introduce more overhead or require a re-architecture of Flash Player.
This isn't very scientific - ignoring cache and lots of other considerations - but memcpy()ing 30fps of 720p video from one buffer to another eats 5%. Decoding h264 in QT, and copying the buffer would take 15-25% total. 25% compares favorably to 100%.
I'm aware that the problem is the way Flash writes into Safari's window. But if Adobe thinks that h264 acceleration would dramatically improve its performance, they'd still have to copy frames into Safari somehow.
(Maybe they mean they want to write directly to the GPU's buffer...)
http://blogs.adobe.com/penguin.swf/2010/01/solving_different...