Just dropping by with my experience...
Had the lag in 10.10.1, specifically, the Mac was fine on boot but degraded over time, after a couple of days the UI got painfully choppy, particularly Mission Control animations.
I thought 10.10.2 had fixed this, but I started getting the same problem again, this time with bad battery life and a hot Mac.
I emailed Apple with specific details, a couple of days later an Apple guy contacted me and said that they realised people were still complaining, and they wanted to find Macs with the bug to do some diagnostics. I said wait a bit, the Mac is fine at the moment, let me see if it happens repeatably, and if it does I'll get in touch.
I did some diagnostics on my own and I found out that the problem was Preview.app. After a few hours use (scrolling large PDFs) Preview's CPU use rises to about 20%, and windowserver's to about the same level. Overall the CPU rises to about 40% even when the Mac is doing nothing. The UI gets horribly choppy again and battery life drops to around 5 hours.
But quitting Preview and relaunching it fixed my problem. I find I have to quit and relaunch Preview every day, or this problem creeps back. It's now about a week since my last reboot and the UI is still fine. I contacted the Apple guy and let him know all this. He passed the info on to the Engineering team, I don't know where it goes from there.
So I would suggest, if you are still having lag problems
(a) check Activity Monitor, windowserver should be 1% CPU or something when idle & look for rogue apps with high CPU use
(b) try quitting & relaunching apps, particularly Preview
(c) get in touch with Apple, with a specific email, as many details as possible
Basically, Apple wants to know if this is a real problem and if so, talk to people with it.
Having said all that, I think 10.10.2 is still a bit choppier than 10.9 and earlier. Particularly Mission Control, sometimes when I hit the Mission Control key first time there is a small hesitation (tenths of a second?) before anything happens. Then sometimes the animation frame rate is not fantastic, especially with >10 windows. Subsequent presses don't have that hesitation. Window resizing, Finder scrolling are not bad. Disabling transparency helps with the frame rate a bit, but doesn't help with that initial hesitation. It's not horrible, it's perfectly useable, it's just not gorgeous.
2013 rMBP, Iris integrated graphics, 10.10.2, upgraded from 10.9
