The size of the library is not critical for the overall performance. You may notice a little reduce in speed, if the projects are gigantic. I would not put thousands of images into one single project.
But it is more important to keep plenty of free disk space on your system drive, and to have the Aperture library on a fast drive with a fast connection. Also fix your Aperture library by repairing it, if you had a crash or you force quit Aperture.
If Aperture is slow, try to find out, what it is doing by looking at the Activity window (Window > Show Activity). There you can see, if Aperture is processing previews, scanning for faces, etc. Sometimes Aperture is processing the same image over and over again. If you see this happen, remove the the image from the library. Probably it is corrupted.
Regrds
Léonie