Different books can have different reading speeds, think of reading light fiction vs a text book, so re-using the speed may not be the best idea, though certainly, for the same book and same user it could be safely re-used.
Different books can have different reading speeds, think of reading light fiction vs a text book, so re-using the speed may not be the best idea, though certainly, for the same book and same user it could be safely re-used.