Edit: After discovering some more about the problem (namely that it seems to be caused by waking from suspend) I created a new question.
In Ubuntu 10.04 64-bit, I was running the latest 64-bit Flash beta with Google Chrome 64-bit (stable) and I was seeing smooth performance (even in full screen), extremely reasonable CPU usage, and no issues with lock-ups. Compared to my MacBook, the experience was remarkably better. It even solved the issue where hitting volume keys would kick me out of full-screen mode.
In 10.10, with (to my knowledge) exactly the same setup, I'm getting ridiculous CPU usage, and full screen performance has gone from perfect to unusable (this is happening because it's flooring a 4-core i5 processor—no small feat). I'm also back to not being able to hit the volume keys without kicking Flash out of full screen and showing a frozen video player until I scroll the window, something that I fixed in 10.04 from overriding the built-in system Flash install (which I think was the 32-bit version with npviewer) with the latest beta.
This is on a completely fresh install on a Lenovo X201 (no proprietary drivers). Anyone have any clue what's going on? This is pure speculation, but it feels like this has something to do with hardware acceleration not kicking in properly (the spike in CPU usage would support this).
EDIT: Some more details: I've noticed that embedded YouTube players flicker annoyingly often, even outside of full-screen mode, which doesn't happen on YouTube pages. Also, even HTML5 videos on YouTube, when put into full-window mode, are somewhat choppy. Movie Player doesn't miss a beat when playing 1080p videos, so it's possible that this is a Chrome rendering problem, not necessarily a Flash problem.