Everything really slow after installing Yosemitie

Steve, what I have had to do is open Disk Utility, select the internal storage, Repair Permissions. DU then returns a huge list of errors, so I repeat the repair until it only returns one or two errors, and then Restart. That seems to clean up operation. Also, you might want to go to System Preferences, Spotlight and turn off the Spotlight Search Suggestions, that seems to load things up. With a new install Spotlight spends a lot of time gathering information give that time to complete its tasks.

Did you activate FileVault when you upgraded?

That's up to you. Do you consider the problem solved?

OS X: About FileVault 2