I suspect that they were overstating a limitation of these devices rather than speaking from inexperience. While it has been years since I've used a Kindle, I do use Kobo devices and the delays are perceptible. While changing a page may be quite quick, user interface elements (such as a box containing a definition) seem to take longer. I suspect that they have to be more agressive when refreshing the screen before and after these user interface elements are displayed in order to make the ghosting less perceptible.

If you want to see what I mean by the ghosting of user interface elements being more perceptible, try using KOReader. The ghosting after using a menu can be quite noticable (at least on Kobo devices, which are based on the same technology).