Every since I upgraded to OS X Yosemite, the experience has been lackluster. A simple Google search will reveal a host of problems Yosemite has introduced, but the absolute most annoying problem for me has been the UI lag. This UI lag mainly occurs when I have an external monitor attached.
What do I mean by UI, user interface, lag? Activating Mission Control is not smooth. The animation is horribly slow and jerky. At random times when switching between windows the mouse will jump. Clicking or switching (with the keyboard) between windows on different screens will cause the menu bar to refresh itself also causing delayed keyboard input. In other words, my actions were faster than the computer could keep up with. This should not happen on any modern Apple computer.
Until Apple provides a real fix, there had to be something that could be tweaked to provide a temporary fix. A Google search reveals a bunch of various "fixes".
First, I tried enabling Reduce transparency in the Display Accessibility settings. Any sort of perceived improvements quickly went away and the UI lag came back.
Second, I tried the steps in David Gyttja's post Fix lagging display performance on retina MacBook Pro. Many blog posts and forums suggested this two year old post as the solution. It wasn't. The user interface lag eventually always came back.
Third, I tried a fresh install of OS X Yosemite. Perhaps something was screwed up because of the upgrade process. Again, the user interface lag returned.
At this point, 10.10.2 was released and I hoped it would provide a fix. It didn't.
Finally, the only change that has appeared to do anything is enabling Increased contrast in the Display Accessibility settings. Not only did I immediately notice a drastic difference, but the UI lag did not return after a couple of hours like it always had before.
Turning on Increased contrast also turns on Reduce transparency and this combination makes parts of the UI look a bit odd. In particular, buttons and text fields have obvious black borders around them. But, this is a small price to pay for finally having a responsive user interface.
Pierre Igot goes into even greater detail in his post Responsiveness problems in Yosemite. As he mentions in his post, the fix above may not work for everyone.
Regardless if the fix works or not, there is a glaring software or graphics problem that I have a difficult time understanding how it got passed Apple's QA process.
Update 2015-03-28: The public beta of OS X 10.10.3 has fixed my issues of the mouse jumping around when switching between windows and the menu bar refreshing itself when switching between windows on different screens. With Increased contrast turned off, activating Mission Control is better but still not as smooth as it should be (even when the system is using the NVIDIA GeForce GT 650M).
Update 2015-09-15: I installed the El Capitan GM Candidate and it fixed every issue above. Even on the new retina MacBook, everything animates silky smooth.