It carbonates faster when cold. Also, you could try carbonating at a higher pressure, say 20psi, to speed up the process. Shaking also speeds things up, but I've had beer get into my gas line when doing that, so be careful if you try it. Just remember to cut the gas and bleed off the pressure before pouring a test glass if you try carbonating at higher pressure. Then when it reaches the desired carbonation level, drop the pressure and enjoy cold, carbonated homebrew.