There's certainly nothing wrong with Shotwell... but if you're searching for something loads a bit faster but doesn't come with a lot of features (don't be fooled, it's quite powerful!) then Mirage is a pretty good one.
But before we go further along this road ;-), just remember, if you have a huge list of albums to manage, then this is not the one to go for.
*. Uses the GTK+ toolkit that integrates flawlessly with the Ubuntu Unity, Gnome classic desktop and other GTK+ based ones such as LXDE & Xfce (heck, you can even use it under Qt/KDE as well). .
*. Supports a lot of file types (including both raster and vector graphics) such as: png, jpg, svg, xpm, gif, bmp, tiff and many others.
*. Zoom/Out, Flip and Rotate, Full screen and a small thumbnail window to the left, etc.
*. It may be a simple app yet you can do basic image editing such as : Cropping, Resizing and color changing too.
*. View image properties.
*. Save in different formats.
*. Take screenshots.
*. Rename, Delete pictures.
*. Image pre-loading (which helps to reduce the loading times).
*. Slide-show support.
*. Change between image scaling algorithms.
*. Change thumbnail size and background colors... man, so much for "simplicity" :P.
You can install Mirage in Ubuntu 11.04 Natty Narwhal, 11.10, 10.10 and 10.04 by using the below command in your Terminal window.
sudo apt-get install mirage
But remember, unlike with Shotwell or F-Shot, etc Mirage is not built to let you easily handle lots of image libraries (because that's not its purpose). Yet, all you want is a fast loading, pretty simple image viewer that has some useful features, then Mirage is a pretty good looking app nonetheless.