Are you running a single 2s, or dual 2s in parallel or series? Also what gearing are you running on the 1/16 E-Revo? Remember the tires on the Slash are a lot smaller, so you have to gear it higher than the E-Revo for to get the same speed. 28/45 on the Slash VXL is the reccomended gearing while running 2x2s in series (4s total). Stock gearing for the VXL Slash is 31/45. Also make sure that nothing is binding, and that your gear mesh isn't set too tight.