without losing FPS
There are multiple web sites which can slow down gif animation for you. But they do it by simply changing FPS, and because of this result looks choppy.
However, by using the following ffmpeg command, you can slow it down without losing FPS:
ffmpeg -i in.gif -filter:v "setpts=2.0*PTS,minterpolate='mi_mode=mci:mc_mode=aobmc:me_mode=bidir:vsbmc=1:fps=16'" -y slow.gif
where fps=16 is desired fps
(set it to same value as fps of original gif, probably),
and setpts=2.0*PTS means "make it two times slower".
If you see visual bugs - try changing parameters of minterpolate. If nothing else helps, check wiki.