On Windows (others too I think), VirtualDub (free) can open Animated GIF directly, and you can “Save As AVI”. Before you do that, you can adjust the frame rate there.
Or save as avi and open in Windows Movie Maker and slow it down that way.
I use very basic tools but they work smile emoticon
VirtualDub also saves to AnimatedGIF although the file size is much much bigger.
So – there’s ALWAYS a way smile emoticon