Reading through this thread it seems like the exact same problem that I am getting. Except that I keep calling the problem "High X11 CPU Usage". The issue that I notice the most is that X11 just goes to pot and the entire system becomes laggy. However, I also have significantly slower VDPAU playback too.
My system, when it starts always starts just fine: no performance issues. But, after some random amount of time (it is never consistent) the system will start performing horribly. And just take a look at the difference.
I do not really know what to make of this but I am attempting to solve the problem as best as I can and I am keeping my thoughts and musings in this Google Doc which I just made publicly viewable
I suspect that all of the issues that say that "X11 has high CPU usage" will also be linked to the "VDPAU performing poorly" issues. Please let me know what I can do to help. I have already provided nvidia bug reports but I can do so again. In-fact next time I might attach them to my Google Doc instead of these forums. (We should have a public bug tracker to keep track of this; did somebody say Jira