[Renate]

Another test to make would be to make the text as small as it goes. That would make the processing more complex but shouldn't make the actual updating any slower.
Then compare that with making the text enormous.

Edit: Hmm, on my reader the actual compose takes 125 mS irrespective of the font size being tiny or humongous.
Note: This is not a contributor to page turn times because it's already done before your finger hits the screen.

For readers that do an actual graphical compose in the background there should be zero difference.