Some people just have slower internet connections. My browser is normally fine, my computer is fine, but I have a hard time with gif signatures because my apartment complex just has an awful internet connection for things like gifs, videos, etc.
650 frames is way, waaaay too much for a forum signature, no matter how compressed. Every person is still going to have to end up loading each and every frame from that gif. Try to cut down on that, please... any gif signature needs to be as brief as possible.