The Kindle 3 is noticeably faster than the Kindle 2 but it still might be a bit sluggish for quickly flipping through pages the way you'd want to when Photoreading. Presumably by the time the K4 or K5 get here in a year or two they'll have improved the speed some more.
I just did a quick test on my K3 and it took 13 seconds to flip 10 pages at the smallest text size, and 9 seconds at the hugest size.
On my iPad the Kindle app, iBooks, and GoodReader (that I use for reading scanned PDFs, as 2-page spreads) each took about 5 seconds to flip 10 pages. Text size (on the former two) didn't seem to make a difference here.
My clumsy flipping through a paper book 10 times (to show ten 2-page spreads, or 20 pages) took about 8-9 seconds.
Text size is variable on ebook readers, so at a small text size you may be able to get a single "page" on the Kindle or iPad that shows the same amount of text as a 2-page spread in a paper book. So take that into account too.
I might start looking at the iPad for Photoreading now! I'd never really thought about it before.