New issue...
This "slow" is probably a setting for momentum in the loco, or the controller or both.
There are reasons to not have immediate starting and stopping, your trains and their geartrains will last longer.
For the acceleration and deceleration in the loco itself, it is CV3 and CV4, you might want a primer on the typical CV's to change....
On the Massoth control system itself, I'm sure someone here knows. I use Zimo.
Greg