The gif class is made by SuperRembo back in 2006, very short and compact. View the source here. The old time favorite, GIFEncoder is still at hand, so I did a test and the result shows SuperRembo's gif class is faster after a little modification(my test could be inaccurate because the structure of 2 gif system is different). Take out the ob_start and other part, let the user input the output image instead of the resource provide better result improves speed a thousandfold.
SuperRembo's gif class could be faster because it uses the PHP built in function pack instead of a massive amount of bitwise operators.
Anyone who thinks they can do a better job, read the gif89a specification. Strange, I don't see anything about animated gif in there.