drawing on my kobo using the Plato sketch program is actually super responsive. I'm not sure why but it seems that the refresh rate is fine and e.g. flipping a page or scrolling takes a long time for other reasons than the refresh rate
In practice you'll see full screen refreshes be the high quality type, and smaller refreshes being of the fast type. That said plenty of e-ink displays don't bother with the fast refresh or even with partial refreshes and just always do a full screen high quality refresh. For E-books that's probably fine.