I see a fair number of people complaining about Mavericks' speed, so I'm throwing my $0.02 in (why not, everyone else is!). I updated my early 2011 13" MacBook Pro (4 GB RAM) to Mavericks, and for the first hour or so... what a dog! It was just unbelievably, painfully, slow. But, eventually, all the background re-indexing and compression finished - now it's running fine. Some people are responding to complainers with a knee-jerk "you should always do a clean install". Honestly, you should almost never have to do that with any modern OS under normal circumstances; but saying that seems to be the latest iteration of sage advice like "rebuild your desktop" or "repair permissions". Just upgrade to Mavericks, be patient, and everything will likely be fine.