It isn’t JMRI, it is that the settings within the decoder are not matched to the loco. It is not unusual for an out-of-the-box loco to reach its maximum speed well before the throttle is at full. What you need to do is adjust the speed curve (either 3 point Vmin, Vmid, Vmax or speed table) such that the loco reaches its top speed (or the top speed you want, if lower) when the throttle is set to its maximum. JMRI is an excellent tool to help perform this adjustment.
OK, will try that. Was not sure if I wanted to play with that feature, as i left everything stock. It is currently using the curve.
