I'm guessing screen refresh involves these three steps:
- Time to process command (ie: register click, open document, access requested content, etc... all limited by cpu, ram, and storage bandwidths)
- Time to pass complete screen image to e-ink controller (in irex's case, the delta controller) and for it to process.
- Time for e-ink paper to update
When you open a document on a traditional display, steps 1, 2, and 3 can start in parallel: as the document is being loaded, the screen can start showing the application interface and when more of the document is loaded, more of the document can be displayed. It's like viewing a web page where things load in chunks, giving you a sense of progress.
On an e-ink device, the above steps usually need to occur in serial (one before the other). For a full page refresh, such as with a page turn or when opening a document, the e-ink controller needs to have all the screen elements in place before it can finally tell the e-paper what to display. There is capability for partial refresh, but that involves changing small parts of the same page and the original image can be degraded in the process. I guess one can think of e-ink like printer paper... you print to it when you are ready.
I suppose if device makers think components that are 'fast enough' for traditional display devices is good with an e-paper display swapped in may need to rethink things. If they can get steps 1) and 2) to be really really fast, then the physical slowness of step 3) will be less noticeable.