The GIF loader basically grinds to a halt then loading largeish files with the progressive API
If the progressive API is used to load GIF images the call to gdk_pixbuf_loader_write() takes longer for each time. For largeish images that may cause thousands of calls it basically makes the loader grind to a halt. The time is mostly spent in memset() called from gdk_pixbuf_gif_anim_iter_get_pixbuf(). This issue seams to have appeared in version 2.40.0 (it was fine in 2.38.2). (It can be avoided by calling gdk_pixbuf_loader_write() just once with all the data.)
The problem affects many (most?) image viewers based on gdk-pixbuf. Some of them, like gqview/geeqie, updates the image while it loads and thus clearly visualizes the effect of the bug (while others just appear to load forever).