This is just one example, so take it for what it is.
I downloaded the book 'Cranford' in ePub from PG to my Sony PRS-505, and could hardly read it because it was taking 2-3 seconds per page turn. I downloaded the same title from the Google-Sony store, and it pageturned normally. It contained a few OCR errors, but nothing I couldn't puzzle out while reading.
I'm new to the 505, only had it a few days, so I have no idea why this was happening.