WindowServer taking 90% CPU for 240p YouTube

[Hamo]

I just noticed that things are looking pretty chaotic in Activity Monitor when Flash content is active in Safari. The WindowServer process is consistently using 90-100% CPU even when playing a 240p YouTube video, the actual plugin and Safari processes are fairly reasonable at around 15-20% but this WindowServer usage is insane.
I know Flash has a bad reputation on the Mac, but things were not this bad for me in the recent past.

There have been two changes to my system in recent days.
One was the update to Flash 11.5.502.136 - is that version known to be particularly bad?
The other thing that happened was a power failure while browsing the web the other day. Could that have done something that is making the WindowServer hog CPU (but seemingly only for Flash content)??

MacOS X 10.6.8, Safari 5.1.7, Flash plugin 11.5.502.136, Mini C2D 2.53Ghz/4GB