What does Aperture's Activity window show? (From the main menu bar: Window > Show Activity)
Is Aperture still busy with upgrade tasks running in the background?
When you followed Kirby's suggestion - did you only run "Repair LIbrary" or also "Repair Permissions"? I'd do that as well.
And i'dtest with a new, small library with only a few test images to see if the library is to blame.