Hello all.
...
I've got the file up to 4.01GB in size with 647 images contained therein. As Brink has said it does take "many minutes" to open the file completely. Fortunately (so far) file size doesn't seem to have any effect on how Desktop Slideshow operates. The only thing I've noticed; if it's on "shuffle" it doesn't seem to go through the entire file; it seems to 'get stuck' around a certain group of images; and continually repeat the same section of the file.
I will continue to keep you updated.
Later
Ted