Hi,
This may be useful to those of us still running the PRS-650 and using PRS+
The background is I use my Sony PRS-650 to read technical books travelling to and from work by train. I find its size, battery life (fingers crossed) and eink screen to be better for commuting reading than my larger Samsung tablet.
Now for the story:
I took advantage of a recent offer from O'Reilly Books and bought an ebook on Software Requirements. I downloaded the epub version onto my PC and then sideloaded to my Sony PRS-650.
Reading this book on the Sony was a dreadful experience - slow to load and turn pages, crashing when I tried to go back to a previous chapter or just freezing.
So I checked the file size - 20MB! and talked to one of the developers at work who said size could be a problem for a low power ereader. As a check, I uploaded the epub file onto my large Samsung tablet and read the book using the Mantano ereader software - no problems, so this pointed to the file size as the most likely cause
I then split the epub (using the ePubSplit plugin for Calibre) into 5 smaller epubs with the largest being 6MB and downloaded these to the Sony.
This improved the loading speed and page turns, but I still got some freezes and crashes. So I did some more digging on the Internet and found that from early 2013 O'Reilly Books had switched all their books to the epub3 format. I could not confirm that the PRS-650 has problems with epub3 files but it seemed a reasonable assumption.
I then checked Calibre's epub -> epub conversion and found the output would be epub2-ish. I put the 5 smaller epubs through Calibre's epub->epub conversion (targeted for Sony eReader).
The 5 epub file sizes shrank by 50% so the largest is now 3MB. The true test was downloading the new versions of the 5 smaller epubs and then reading them. Speed is good, freezes are not happening, looks good so far (1 day).
It is a nuisance having 5 files instead of 1 - However I did not have to buy a new tablet or ereader.
Cheers
Paul