This is a really great image viewer, and I appreciate all the effort you've made to make it as clean and simple, yet powerful, as it is.

I've started using it to view very large sets of images (20,000) and the performance upon opening the folder is really bad. The source of the problem seems to be that when you sort the files you call CFURLCreateFromFSRef for every single comparison operation. Maybe instead you could call it once for each file and cache the result? Or even better would be to build the full expanded path when you first read the directory contents so you don't need to expand it later.