I own both a Kindle 1 and a Sony PRS-505. It took me a very short while to adapt to the speed of the page turn. When I'm reading, I will press the page-turn button at exactly the right time to allow me to finish reading the last line(s) of the page and then continue immediately on with the first line of the new page. This "anticipatory" page turn reminds me of sitting on the piano bench during a concert and turning the pages for the performer.
But ... to answer the question you asked, I believe that the fastest page turns are from the devices that do not have an e-Ink screen. Jetbook and eBookwise are two that come to mind. The wiki lists these two and a "Dr. Yi" as the only non-e-Ink readers which are still being manufactured. (See
https://wiki.mobileread.com/wiki/E-book_devices)