Scrolling slowly using a 3rd party mouse does absolutely nothing on Sierra for me. I just now noticed after using the trackpad for several months. I even purchased a new mouse to see if Logitech's software was interfering with something to no avail.
I am using a Razer Naga, but the same issue occurs with my Logitech MX Anywhere 2, MX Master, M705, and my generic Microsoft mouse.
Here is a little video showing the issue in action: https://www.youtube.com/watch?v=uB975jLpmKg
I have seen one other person with the same issue on Sierra, but zero solutions unfortunately. Please do not recommend USB Overdrive or SteerMouse.