Interesting. I've recently downloaded quite a few images to use as mood board content or inspiration, or because they are cool, or because they have problem for me which stands out and I keep them to show models how not to pose etc. 'Dwell time', as I believe it's called, was about 3 seconds, before I clicked on to the next image. And I emphasis these are images I like enough to download. Looking at photographers' portfolios, I would imagine 1-2 seconds per shot. Perhaps 5 if it grabbed me.
But is that comparatively short time a reflection of the quality of the image, or the way I work? I view many model portfolios a day (somebody has to, right?) and on the hosting site I visit most frequently, I take less than 5 seconds to scan the 20 thumbnails on a page and I know if I'm interested in working with that model or not. I have assessed in that time, her looks, the way she moves and the quality of the photograph (to differentiate between a good photograph and a good pose).
I'm also interested in antiques. I can spend several minutes looking at a piece in a Bond Street dealer's window. Yet when I go to car boot sales, I walk past most of the stalls and scan the entire table while walking past. But I like to think that if there was something there for me, I'd spot it, even at a slow stroll.
So, my long-winded theory: it's as much about visual processing speed as it is appreciation of an image or an artifact. But for some reason, I read really slowly, can't scan and have little recall. Perhaps I'm just a visual guy.
Incidentally, the dwell time at an adjudicated art (or photography) show at the initial selection stage for a group of up to about 6 jurors is... less than 5 seconds per picture.