I have noticed that single core machines cannot play yt videos well anymore.Try to disable hardware acceleration in flash.
Yes! This is what makes me angry. Prior to the time Youtube went Hi-def, ALL 480p videos played absulutely great on my computer - using around 60% CPU.
Then they switched to allow HD videos. My machine can't handle 1080p videos and is choppy at 720p videos. Fine - I can accept that and throttle them down to the 480p setting. BUT - now even playing those HD videos in 480p mode takes 75-100% of CPU.
If you can find an older video on YouTube that wasn't uploaded in HD then it will still use the older player and consumes lower CPU cycles.
It all seems to be based on a change to the YouTube player. The older videos have a thicker looking progress bar at the bottom (lower res) and these play well. The newer videos (all HD videos) have a thinner progress bar at the bottom and use more CPU even when throttled down to the lower 480p resolution.
So it's frustrating that CPU usage should go up for exactly the same resolution (480p). Seems like a step backward. My machine can just about keep up with 480p now, whereas before it could handle 480p Flash video with CPU to spare. Mostly it plays fine, but now I hear the CPU fan whirring up to high gear, and occasionally there is stutter or the sound is delayed from the video. Seems like Flash is getting less efficient with every version released. Why should a newer version of Flash perform WORSE than an older version?
PS: I have disabled hardware acceleration in Flash. It doesn't change anything in regard to CPU consumption on my machine.