According to the E-Ink Pearl datasheet (Link:
http://www.eink.com/sell_sheets/pearl_spec_sheet.pdf) refresh times are between 120 msec (1 bit) to 450 msec (4 bit) which gives a max refresh rate of 8Hz
I could have sworn I recently seen one giving figures of between 4Hz and 20Hz, but can't for the life of me find it again...
There's a 2011 video of an 8Hz E-Ink display in action here:
https://www.youtube.com/watch?v=0DVlGZ3_d3Q
A hacked Sony Prs-1 here:
https://www.youtube.com/watch?v=ZXYHvYo5LEE#t=31
And video on E-Ink using the Freescale IMX50
https://www.youtube.com/watch?v=7dffqB9PfKY
What kind of features do you think these speeds could enable?