I think I solved it on my MacBook Air 11" (2011)
I have been astonished by the lag, choppiness and instability that I experienced after installing Yosemite on my 2011 MacBook Air (11"). I recognize most of the problems described in this thread, e.g., lag when entering mission control, lag when apps show save/open dialogs, lag in the system info, lag when scrolling in folders in the dock. Additionally, I have seen annoying lag when just moving over my (auto-zoom) dock icons, and unacceptable choppiness when swiping between desktops and full screen apps.
However, swiping between desktops on an external screen while keeping an eye on activity monitor on the other screen learned me this: It is all because of WindowSever. WindowServer peaks to 90% CPU usage when doing anything graphically "complicated" like swiping between desktops. And it virtually never went below 40%. For sure, it never left the top 3 of most CPU-heavy processes.
The problem seemed to gradually build up: after some hours/days of usage things got worse and worse. (Total Windows 98 ME style I would say when feeling naughty.)
But now I have turned off translucency effects, everything runs smoothly again. I am almost surprised but happy to see that WindowServer is sitting somewhere around 1-2% CPU all the time.
So, I think I have solved it, but I also think that this is unacceptable for Apple.