Thanks to http://blog.floriancargoet.com/slow-down-or-speed-up-a-gif-with-imagemagick/
identify -verbose your.gif grep Delay| give current delay between each frame in 100’th of second. So it it say 50x100 then the delay is half second. To change the delay to one second between each frame do
convert -delay 100x100 your.gif your_slow.gif
So to make the time 0.25 second between each frame do
convert -delay 25x100 your.gif your_slow.gif
It looks like 6x100 is min time betweeb frames that browsers support. But this could depend on which browser.