I'm having the same problem generating thumbnails (only I'm using Magick::Image rather than ImageList). I've tried a bunch of stuff but nothing seems to work so far. Incidently the problem seems to occur regardless of the image size.
I'm probably going to resort to one of the following: - a spinner gif and an ajax request to see if the image is there yet or - checking if the file is on disk before returning the generated thumbnail path (and manually blocking the return until the file is written) both options make me cringe a little though....