There are a lot of good answers already. I would like to add on to them the fact that eInk screens are not inherently slow. There was this Chinese company around 2010, that made eInk screens that can show 48fps videos -- however, they were not able to advertise themselves right, and they went bankrupt; their technology was bought out by Sony I believe for around $1.2M, and then promptly discontinued. One example of a similar quick video playback on an eInk screen is https://www.youtube.com/watch?v=24srQXX81Oc (though that's not the one from that company).
The idea is, that the lag that's incurred is happening because the USB interface is using the "naive" way of sending of the image. The actual update speed of an eInk screen is close to 300 Hz (as measuring using an oscilloscope), but you have to update the screen in the correct order to remove the aliasing effect. Therefore, most eReader companies simply flash the entire screen black, white, black, white, and display the image -- which works, but it makes the image update very slow. If the USB driver is programmed right, eInk is capable of very quick updates.
One company that sells relatively quick eInk monitors which is not bankrupt right now is Dasung. In the 3rd generation of their displays, they have been able to reach approximately a 20fps update rate (with the exact rate depending on the complexity of the image). So eInk is not inherently slow: it can definitely do animation. But since people are buying them the way they are already, there is not enough incentive to really put in the development to make them display animation quickly.