Macbook Pro with Yosemite very slow
It seems to be a common issue with Yosemite, see e.g. a Mac Rumors thread.

In the last couple of days there also popped up several articles complaining about Apple’s current state of its software. You can find a good overview of the most common issues in this article (it’s also worth to skim over the comments as there are issues that aren’t discussed in the article).