Originally Posted by mishaikin
645 kB - it doesn't seem so much. I tried another epub, no drm, 311 kB - less than 5 seconds to load.
650KB by itself shouldn't take that long, DRM or not, so that leaves what I said; structure.
One other thing I've noticed, ePubs assembled from many linked files seem to take longer to open than epubs built off a single file of the same overall size.
To go deeper than that you'd have to strip off the DRM to see how much faster, if faster at all, it loads. Also DRM-free epubs can be renamed as zips and their structure inspected to see how they're built.
An "interesing" puzzle...