Problem: My iPhoto performance has been degrading over the past few months. Scrolling, editing and browsing became harder and harder to do.
Solution: Hold down the Command and Option keys at the same time when opening iPhoto. You will then be given an options to Repair Permissions, Rebuild Thumbnails, Repair Database and Rebuild Database. I Repaired Permissions and then the database. iPhoto performance and startup performance improved a great deal.