Looking further it does seem that most lines have a 60ms pause as each end. The first line seemed different for some reason.
Here is graph that goes right up to full speed where it starts engraving. To get an idea of the real speed I used a one tap IIR filter to average the dithered speed. That is the red line.
It looks simple harmonic motion (like a pendulum, which is the smoothest form of motion) except for the pause.