Hi Westong
I found a way to slow it down.
You need to use GIMP
http://www.gimp.org/ It requires X11 environment which you should have on your Mac.
Start Gimp and then load your image, save it and when prompted for "export" check "animation" and then you may choose the time lapse between frames. I chose 110 ms.
Once done, you load the new .gif in Keynote, it should play slower than before.
Hope this helps you.