Is there any statistics on how long it takes to generate a PDF, e.g. high resolution, with 3-4 300x300 dpi tiff images? I'm trying to determine if it warrants a "publishing" popup for the UI, or just let it publish.

I would recommend testing this yourself, as it really depends upon the speed of the server and the complexity of the document. If the images are local files it shouldn't be more than a couple of seconds.