When I first got my rMBP, I migrated all my data from my older Macbook using Migration Assistant. My older macbook was a 2011 MBP running 10.8.5. Everything worked very well after that. The UI performance - of which I was really concerned - was perfectly smooth. But then, I decided to to a fresh reinstall Mavericks. My UI performance is noticeable worse. Using Quicklook, the animation shows particularly low framerates. Why is this? How can doing a fresh reinstall actually worsen performance?