It's likely that you just need to fiddle with how long each frame is supposed to be displayed for. It's kind of slow by default.

If you click the "..." in the upper-left corner of a frame you will see the option to specify the delay speed. And you can alternate between "current frame", "selected frames" and "all frames" which is useful if, for example you want to set the same delay speed for all of your frames.

I hope that helps.