Power (amps) for torque or strength and potential (volts) for speed work hand-in-glove, so be sure your DCC system has enough of both. DCC starter sets are often designed to provide enough for n and HO scale. You may need to 3A or 5A boost one for dependable S running. I prefer 18V over 15V. DCC transmits signal through wheels in contact with rails (not through the air as with TMCC and Legacy) so be extra certain both are very clean and that you're getting consistent power delivery around your layout. Use a DCC meter under load (~1.5A with a light bulb) to measure DCC voltage to the rails as you drag it along your tracks. Address breaks and/or drops in power. Enjoy!