When Amazon rolled out the 5.10.3 firmware update earlier this week, all they told us officially was that it was new and free. (Usually the changelog at least mentions "bug fixes and performance enhancements", but not this time.) But according to users over at MobileRead, the update did actually enhance performance:
I just updated my Oasis 2 and the only thing i am noticing is much faster responds on opening books, and page changes.
The update has been released for all current models of the Kindle as well as the Voyage, the older basic Kindle, and others. Basically every model from the second-gen Kindle Paperwhite has been updated with these performance enhancements (I checked).
Amazon will roll out the update over the next couple months. If you do not want to wait, you can visit Amazon.com and download the update, and then install it yourself.