You may have been advised to clear the Aperture cache. This can in general speed things up, but initially, while the cache is rebuilt, things may actually be a little slower. This is a basic “good housekeeping” step. If you’re using an app like CCleaner [Free, Mac App Store] it would throw this folder away for you.

The cache folder to delete is located at ~/Library/Containers/com.apple.Aperture/Data/Library/Caches/com.apple.Aperture

Simply quit Aperture, trash that folder, and relaunch. 

Special Note:

You should not remove the cache in the System Library (that’d be Macintosh HD/System/Library/Caches/), specificially the file com.apple.xpchelper.cache as that may break video playback. Notice this is a different location than the instructions above, so as long as you follow the steps above, you will be fine. There are instructions elsewhere on the web to remove the System Library Cache, but the results of this are Not Good™.