Are you putting the Macbook into clamshell mode? When I hook up my Macbook to the TV, I load up the video player, then close the lid so it outputs only to the TV. I think if you have the lid open, it's using up processing power for both screens.
I have a black Macbook C2D 2.0 GHz and it runs various containers up to 720p (1080p - forget it) smoothly on my TV.
BTW, there's an update for MPlayerX v1.0.8 since Aug. 18th.