
I won't argue your logic in respect of pre caching images as you are the ones kind enough to develop a great package.
I use a qnap NAS on my home network as both a NAS and a webserver.
My example would be that I attended a 40th birthday last night and saved the 167 images after editing in my photo folder. Previously I would have pre cached the images prior to announcing their availability to other guests. Once pre cached it was a swift process for page loads.
You are correct in that now it appears I have to manually look at thumbnails and wait for their generation (about 20secs per page) and then select each to view/create the mid size image (about 3 secs each). The alternative is to create a 2nd copy of all images on the NAS and resize them all to a smaller size (in my mind this is needless work and a waste of space as I would have 4 copies of each image in varying sizes instead of 3).
This is down to me and my requirements and zenphoto with pre cached images is still the best solution I have found, so for now I will manually pre cache by browsing the galleries before telling folks.