XnView Software

In the main window (i.e. images list), "scrolling" over large images with the keyboard (i.e. either by holding the up/down key, or by pressing it repeatedly) is very slow. The reason seems to be that XnView is decoding each image, before advancing the cursor bar to the next image.
My idea to make this faster: Before starting to decode an image, check if another keypress was received. If yes, don't decode it at all, and just move the cursor bar.

Thank you, Thomas, for your suggestion. AFAIK, XnView has various speed optimizations including your suggested "check for additional key press". By default, XnView caches generated thumbnails so the problem that you experience should only happen the very first time you browse a folder.

I assume that you know that there is "Page up" and "Page down" keys for pagewise scrolling. Though, I'm not quite sure whether this will be much faster in your case.