I've noticed on Channel 9 in the past that going full-screen would cause my CPU fan to ramp up because the Silverlight plugin was using extrodinary amounts of CPU to play the same resolution video that Flash played easily. I didn't think much of it because I guess even the highest resolution videos on Channel9 are fairly easy going and my desktop PC (Core 2 quad 2.4GHz) at least could play them OK.
But a few months back Lovefilm switched from Flash to Silverlight for it's streaming video service (yeh, just as everyone else was figuring out SL had no future). And more recently they started offering streaming in HD. And the bad news is it switches streams on the fly so when it's boxed in on the webpage it stays SD, but when you go full-screen is automatically switches to the HD stream.
Problem is, it uses nearly 100% CPU when playing HD video and so the sound breaks up and the video stutters. Again, Flash plays equivalent bitrate video from Vimeo just fine. If I bring up the setting panel for Silverlight it says hardware acceleration is enabled. I'm tempted to think there's some quirk of the videos they are streaming at lovefilm that means it doesn't use hardware to decode them, but then I remember how it uses nearly all the CPU time on Channel 9 too and if Microsoft can't get a Silverlight site right, who can?