I held a colour 60Hz electrophoretic display a decade ago (first startup). Electrophoretic displays can go at very fast refresh rates if you just zap them hard enough (i.e. high voltage on the TFT). Without higher voltage they can still run fast but at the expense of losing contrast. You basically have little black and white "balls" (electrophoretic particles) travel up and down in the field (in those little e-ink cells). At conventional voltage the balls won't travel the full distance in a single fast image frame, so you won't get full black or full white. The problem is that people want colour, speed and low power consumption. Those are opposing requirements and E-ink made their bet on power (at the expense of colour and speed).