[GraemeJ]

PDF quality is a lot better, but file size is almost the same - around 800 KB.

Hmmm... I think this may be at least a partial reason for your problem.  Most of my PDF's are 100 KB or less.  I have a couple that are larger and I checked out the load time of the biggest one I could find (222 KB) and it is still quite quick - certainly less than a second (and that's a lot quicker than it would have taken me to find a piece of sheet music )  - with subseqeunt pages being as near instataneous as not to matter.