Strange, I've only experienced this slow down the other way around; the animation runs slow in the preview window, but fast once viewed in a browser.
I do know that animations run slower in Imageready when there are a lot of frames that don't fit all at once in the animation window, a reason why Imageready has to scroll the frames left all the time when you play the animation, which slows down the animation in the preview window.
At least that's the case on my rather slow computer.
People who are not aware of this slowdown in Imageready, will therefore be surprised to see their animation going faster when they preview it on their web site for example. It is important to preview the animation in the preview window only and not in the animation window that contains the frames.
To speed up the preview you can;
a) resize the animation window so that it shows all frames at once
b) or start the animation and then close the animation window (not the preview window)
I also have to mention that some sometimes I noticed speed differences while viewing the same animated gif in different programs (browsers, image viewers), so that might be the case too.
I'm not aware of any special animation settings, other than the delay that you can enter for each frame. Very large gifs might also have an influence on the overall speed.