I should have mentioned that I'd be using text-mode mostly. With my keyboard refresh rate (25key/sec) => 40ms max per redraw. That's .. quite far. If you look at this old kindle refresh rate, used as a terminal screen for a raspberry pi, it's almost usable : http://www.ponnuki.net/2012/09/kindleberry-pi/
I wonder how it would be on a newer eink screen.