I finally updated. Did a clean install, haven't added any documents back yet, just a few apps. During the installation, everything was really jerky—like the computer is when you boot to Safe Mode and it's not using the GPU. The welcome screen, for example, where the words fly by in different languages was chopped in half.
Now that it's installed, it's not like that, but it's not smooth at all. Doesn't make any difference whether transparency is reduced or not. I notice it when dragging windows, scrolling in Safari, and especially when moving between desktops (only 2) in Mission Control.
I wonder if it's a window server issue. I also notice that when I use screen sharing it takes up 42% CPU versus only 3-5% CPU in Mavericks.
I was expecting it to perform decently as I did a clean install and have avoided any plug-ins (Silverlight, Flash).
My Mac is a late 13" retina MacBook Pro, 2.4 GHz Intel Core i5, 16 GB RAM, Intel Iris 1536 MB.
I feel like I'm using an older, slower computer. It's not awful, but it is noticeable.
I was used to how smooth scrolling was in Safari--Apple had really improved that a lot in Mavericks.
I wonder if this can be optimized, or if OS X requires more powerful Macs than have been released yet to run well.
Update: Right after I posted this, I disconnected the computer that was sharing my rMBP's screen. I tested Mission Control again, and it was very fluid and fast. Then the whole computer froze. I've had the computer lock up in Yosemite quite a few times, but this time the cursor wouldn't move and caps lock wouldn't light up. I restarted the computer and it's still much more fluid than it had been.
Definitely some sort of screen sharing/window server issues.