Have you tried tapping on the bottom left corner of the screen while reading a book? You can switch among various options:
%, time left in chapter
%, time left in book
blank (ie, your current state)
At least the following are supposed to work, although I haven't verified them:
;debugOn // verbose logging
;debugOff // non-verbose logging
~usbNetwork // starts a Dropbear SSH server
;ReadingTimeOff - switches off the reading time display
;ReadingTimeOn - switches on the reading time display
;ReadingTimeReset - resets the reading time computation
Basically, they have no limit on how fast someone could actually read (supporting speed readers) but they do have a limit on how slow someone can read. That is, if someone takes 30 seconds to read a page, they will add that to their calculations, but if someone takes 5 minutes to read a page but only 20 seconds to read other pages then ignore the 5 minute ...
According to a Kindle forum thread the command to reset this data is:
On a touchscreen Kindle, tap the top of the screen to bring up the search box. After you type the command and get no results, return to the book. The status line will show Learning reading speed... Since the search string is saved, you won't have to laboriously type ...
The commands ~ds and ;411 worked for my kindle paperwhite 3. The ;411 command even measures the temperature of the display! To re-enable screensaver for ~ds command, you must reboot the kindle paperwhite a few times. Mine took two times.