Why cannot it be set higher than 9 ?
On my rooted Sony a no-refresh mode is available, which would translate into a so far non-existent "Never" setting on the M92. On Kobo readers you can set it up to 99 in the official firmware.
The flashing of the screen of e-ink readers has been proven by French neuro-scientists to distract the brain, decrease understanding and memorizing of the text. Of course this is not perceived by our consciousness; it takes place unconsciously, like 90% of what takes place in our brains.