Feh

Feh is a extremely fast and reliable image viewer that uses imlib2 to handle images, it can handle all popular image formats (see imlib2 for a list on file formats it can open), and contains no clutter (just the image) in its window.

Feh is a bit command line driven, all the options must be passed to it when the program is started from the command line, as once it opens the images, all it can do is randomize them, or sort by name, delete them, or use them as the background.

My personal experience with feh proves it can easily open a directory of images (over 19,000 images) in seconds (if sort by name, or randomize is used), to 10 minutes (for more fancy sorts, like size), and still never crash.