It also caches one image behind, if you're going backwards. I like to call it predictive loading, and it's a brilliant idea.

To clarify what WAHa is saying in regard to zipping forward (which I often do): If you're stuck with a non-caching viewer, zipping forward is low. You move to the next image, wait for it to load, recognize it, then move on. It takes me substantially less than one second to recognize an image, or scan for some relevant detail I'm seeking. So, I'm wasting at least 2/3 of my time waiting for the damn image to load.

Or, I can use a more intelligent approach: while I'm scanning the previous image, load the next. Transition is instantaneous. Repeat. This combined with ACDSee's wonderful speed means I can scan through many images a second.

You could look at the thumbnails too, except that thumbnails often contain too little detail for what's going on... like, say, manga, or other images that differ primarily in fine detail. I actually detest thumbnails, because with ACDSee they're not even necessary. It's faster for me to focus on one point on the screen and flip than it is to move my eyes (experiments to this effect indicate I'm not unique either).

Once you get used to it, waiting pisses you off.