Just got around to installing the 5.10.3 on my PW4, on this device I don't note any improvements in page turn speed or performance generally. Main things I noticed were small but useful enhancements:

* % of remaining battery life is now available.
* The menu with page fonts and style information has been rearranged, it's easier to use and includes a few new extras, like the ability to create named "themes" which are just a collection of styles. That's a handy feature: I created one theme for programming books, another for poetry; will probably use that a lot.

Will be watching to see if it does anything for battery life. But battery life on the PW4 is already not bad, esp in airplane mode.