The default image viewer on Ubuntu is Eye of GNOME (EOG). It appears with the generic name Image Viewer in the menu. It can handle most image formats and is a good default image viewer. However, it takes several seconds to load up the first time it is used or if it is used after a long time. I find this highly irritating and started searching for a replacement.
GPicView is a simple image viewer that is built purely for speed. Whether launched the first time or anytime, it loads up in a flash with the image. No matter what the size of the image, this viewer is just super fast. Among the many image viewers I tried for their loading speed, this turned out to be the fastest. This is now my default image viewer 🙂
To install GPicView:
$ sudo apt install gpicview
Confusingly, GPicView too appears in the menus with the generic name Image Viewer. It has a different icon, so that may help you distinguish it.
Tried with: GPicView 0.2.4 and Ubuntu 14.04