Kindles refresh the black layer much faster. I don't see why it's soo much slower.
But since it's literally particles moving inside the display, it will never super fast but 8 seconds is very limiting.
I just bought the B/W version of this screen and read the data sheet.
Internally, it is four different screens. So I think (haven’t yet received it) each quadrant takes about 2 seconds to refresh (2 seconds is a pretty common refresh time for eInk). A better driver could do all quadrants at once for a ~2 second refresh.
The way multicolor e-color displays work is by having multiple size/weight particles that are moved by different field strengths, but require being even more careful (because every field, shaking and moving, will move every particle)
Also count their low-power consumption and you can easy convinced even fans of LCD displays.
That said, having a display that refreshes in seconds vs single-digit milliseconds works to keep it niche.
At 0.0625 FPS, SPI bandwidth  is plenty fast with some to spare.
Sony makes a nice one as well, but it's a lot more expensive and a lot more locked down.
These Waveshare displays are more suitable for Arduino projects.