
An animated GIF is actually many images saved in one.
To demonstrate how GIF animation works I shall explain how this image of 2 angry ducks was animated.

There are 2 frames:
Frame1:
and Frame 2: 
which are drawn repeatedly every 1/5
A more complicated effect has been created by this animated LOGO showing a flickering fire
However the only difference between the angry ducks and the fire is that the latter has 10 frames which change every 1/10th of a second. When the animated GIF is loaded onto a webpage, the browser simply loops the images. This means, if you make an animation of a clown that moves his hand up and down in 25 small movements, then the animated area of the image is saved 25 times. So it doesn't take a lot of animation to create a GIF image that's remarkable bigger than a regular clown image.
Therefore, even small animations take forever to load