I don't remember if I actually sent this in.
If you right click on an image in the viewer and accidentally hover over Run Program (on the selection), it is extremely slow.
I tracked that down to the Run Program code looking at all the files to determine the set of MIME types in order to select appropriate programs to populate the menu with.

If we make the assumption that MIME type is determined solely by the extension (which is probably reasonable for this purpose), we can speed matters up tremendously.