Hi Steve. You might check your motion menu to ensure it is already transcoded prior to bringing it into Encore. If you have not exported it as an MP2 DVD file then my guess is that Encore is first taking the extra step to transcode the motion graphic before playback. ....which, depending on the motion gfx, will certainly add a rediculous amount of time to preview.
I've only ever imported motion gfx menus as MP2 files, and have never had to wait any longer for playback than one would a normal menu. Give it a shot and see if that doesn't fix your problem^_^
