I've had serious performance issues with Yosemite. For one, dragging around windows became extremely janky; this appears to be caused by the transparency (and the limitations of my first-generation Retina MBP), as the option to disable it fixes the jank. For another, opening a new tab in Safari started lagging about a full second before I could start typing in the address bar, which is a ridiculous amount of time compared to the previous version or competition. I was able to fix this by disabling Top Sites, but while I don't actually use Top Sites much, I never had to disable it before, and most users won't know to do that, making it a serious regression. (Chrome actually takes quite a while to load its equivalent - I don't know whether it's for the same reason or not - but it does it asynchronously, so I can start typing in the address bar almost immediately.)

On top of that, there are serious performance issues with the built-in Japanese IME, although I'm not completely sure how much of this is new, and I experienced a bug where WindowServer would randomly start hogging CPU, which may or may not be related to said IME.

Given all these problems, I'm surprised that the general consensus seems to be that the performance is good. But maybe I'm somehow a special case. (The Safari issue is the most egregious - is it that many OS X users don't use Safari in the first place?)