> Things as simple as font resizing are 30x slower than on a Kindle
It's not a constant factor. This is best understood as a bug. Here's what the reviewer demonstrated: If you change the font size, then the interface locks up and you can't do anything, until the backend has generated new page images for the entire PDF—no matter how many pages there are. He showed that, while it was fast for a small document, it locked up for about 50 seconds when resizing the font on The Count of Monte Cristo (a large novel, perhaps 1200 pages). Furthermore, he said that if, during that time, you changed any of the other settings (resizing again, changing line spacing, etc.), it would take another 50 seconds, and another if you pressed two of the buttons, and so on. It is very clear that no QA person (whose reports are heeded) tested out those operations on a decent-sized novel or technical manual. (If they tried Harry Potter and the Goblet of Fire—not the largest book in the series—then, going by word count, it would have locked up for 20.5 seconds.)
A sane viewer app—one of which the reviewer demonstrated—would redraw the current page, then give control back to the user, while continuing to redraw other pages in the background. It's a bit tricky, because until you render the previous pages at some level, it might not be clear where the page break for the current page would be; but you could choose an arbitrary point, use that temporarily, and eventually correct it when rendering catches up.
The reviewer did show convincingly that the viewer app is very lacking in features or basic polish. (Another example: You can zoom into a page, selecting a smaller section of it to view. However, you cannot drag this view around the page; if you want to see elsewhere on the page at the same magnification, you must undo the zoom, then select a new region to zoom into.) This is odd for an e-ink device, whose homepage has a section titled "An eye-friendly reading experience: Comfortably read PDFs or ebooks for hours on end without backlight, glare, or eye strain."