An animation can contain several layers and so its size can be
important. This is annoying for a Web page. The Optimize filters let you
reduce this size. Many elements are shared by all layers in an
animation; so they can be saved only once instead of being saved in all
layers, and what has changed in each layer can be saved only.
GIMP offers two Optimize filters: Optimize
(Difference) and Optimize (GIF). Their
result doesn't look very different.