The secret here is to go to the Grimaldi's in Manhattan (20th and 6th). I have been there many times and have never once stood in line. Whatever the delta is between the Brooklyn location and the Manhattan location, it surely isn't 30 minutes of standing around.
Last I heard the difference is a coal-fired oven in Brooklyn, which is not allowed in Manhattan beyond the existing grandfathered ones.
Which Grimaldi's is it again? Famous Grimaldi's? Original Grimaldi's? Famous Original Grimaldi's? World Famous Grimaldi's?