I am not sure why, but it appears that after prolonged or heavy use, Aperture Libraries can bog down. The symptoms range from slow scrolling to occasional thumbnail rendering and overall slower performance. When this happens, rebuilding the Library is often the fix.
Before you rebuild your Library, back up everything to your Vault. Then, quit Aperture.Press and hold Option + Command and launch Aperture by double clicking on the application icon. Follow the prompts to start the rebuild.
It will take some time for the rebuild to be complete. I have my images stored in multiple Libraries of 10,000 image. Rebuild time is about 45 minutes on average.
At the end of the process, you should see snappier performance. This can also be a fix if things are just going haywire without reason.
When you are confident that you haven’t lost any images, back up this newly-rebuilt Library to your Vault and you’re finished.
I recommend that you do this every three months or so and more often if you are a heavy Aperture user or have very large Libraries.
Aperture Tricks - All About Apple Aperture 