From what I've read, though, most of the lag/slowness issues with eink are due to bottlenecks in the supporting electronics, which have relatively narrow channels to the display, and not a lot of processing power. Apparently the focus of most current eink tech is (understandably, given the established markets) power saving and cost, with update speed losing out.

I saw a video a few years ago, where someone had bypassed all that, and was running an eink display at video frame rates. It seemed to work fine.

[...google search...] Ah, here's the video: https://www.youtube.com/watch?v=24srQXX81Oc

There are other similar videos, by other companies, too.

So for a device where more power consumption is acceptable (compared to a power-sipping e-reader), fast-update eink displays may well be possible, and even practical... but it's not so clear anybody is putting much investment in such a thing...