As Melinda mentioned, scrollInertia does the trick, but instead of turning it if it's completely off, you can tweak it to make it faster, but still smooth. Disabling completely made the scroll too nervous of my own free will and made her jump in many ways. This made it impossible to access certain parameters.
I found that 60 ms is the perfect setting:
$(".mCustomScrollbar").mCustomScrollbar({ scrollInertia: 60, });
As the docs say:
Set the amount of scroll as the duration of the animation in milliseconds. A higher value is equal to a large scrolling pulse, which translates into a smoother / more progressive animation. Set to 0 to disable.
source share