I agree with Matt to watch the sea state on either tack...perhaps you're getting the steeper side of the waves on one tack.

Other things to check:
Are your boards/rudders in good condition? Perhaps one side of a blade has a poor repair job and thus you're loosing lift (or just getting too much drag) on one tack versus the other. Have you trued your rudders?

If all else fails, what side of the equator are you sailing on? Could be Coreolis force!