What's the Fastest X11 Image Format?

Spirilis wrote in with this query: "Hi, I was wondering what the fastest image format that can be used by X11 is, or more specifically, with Imlib and gdk_imlib. Reason is, I'd like to take my favorite GTK theme and change all the images to the fastest format that X can handle, so gtk-based windows open up quickly; I've noticed with some themes just the creation of windows is viewable as it draws all the images and stuff. For that reason I'd like to convert the images to something more efficient. I'm thinking PPM, or maybe even XPM, but definitely not a compressed format like PNG or GIF or JPEG. Any ideas?"