they are stored on a external hard drive, my default contact sheet sort is filename and it varies on how many could be a hundred or a couple of thousand
And what part is slow? The initial scan of the folder (whereupon the blank thumbnails start to appear), the rendering of thumbnails, rendering of previews, scrolling around in the contact sheet? All of the above?
The more details you can furnish about the problem, the more quickly and likely I'll be able to help you.
-Kirk