To improve shifting performance of the rear derailleur, loosen the B screw as much as possible. To do this, put the chain into the lowest gear combination (largest cog, smallest chain ring) and loosen the B screw while back pedaling until the jockey pulley starts to rub the cog. Then tighten the screw back in just until the rubbing stops. Most manufacturers set the B screw in too tight when the derailleurs are new, but loosening this screw in effect raises the point of chain deflection making it easier to derail the chain to the next cog.