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A button can wake the CPU directly (the ereader sort of sleeps till you turn a page). The capacitive touch is a subsystem that is probably powered up and polled periodically by the CPU, then the CPU SW has to interpret the information to decide if it goes to menu mode, or changes screen brightness or highlights, or opens dictionary or turns page.
The two buttons probably connect direct to the CPU and likely cause an interrupt. Thus the page turn routine is directly run. Then the CPU goes "sleepy" again.
You'll save power by having smaller margins and font, thus less page turns per book.
It would be surprising if the swipe or tap on screen to page turn (swipe only setting might be faster than swipe or tap?) was as fast or faster than the buttons.
However I've only found the Libra H2O totally reliable on the buttons. The Libra 2 is less reliable and the Sage is worst, sometimes skipping a page. So for reading epub and recording Audio Narration I can only use the original Libra (Libra H2O). Swiping or tapping is more awkward and too slow. I have to remember the last few words on the page as I turn to keep reading smoothly.
I've used an auto scrolling screen (horrible on eink) and autocue, but that really needs an assistant to adjust the speed. There is no perfect scrolling speed for out-loud narration.
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