> it has to redo this how often?
As often as the font size changes.
So, never for most reads? :)
Even the few times I do change text size on my e-readers are largely mistakes. Having gestures to resize is frustrating, in the extreme.
Eh, I don't really have a dog in the fight. When I'm out and about I just read on my phone, which my oral surgeon says is too small for my eyes; I haven't asked my ophthalmologist for advice on my dental implants, but I have been reading as much off screens as off paper since the late 1980s, so any kind of sense I might once have had for the aesthetics of typography must surely have been strangled in the crib.
When I'm home, I read books.