The code changes are all in the Adafruit_I2C.py module. The version can also step significantly faster, but the jitter does start to build up at really fast step rates. The step rates of the varying speed motor here are twice as fast as the previous version, even when the second motor is stepping twice as fast as before the jitter is far lower for both motors. When the steps to the 2 motors coincide it impacts one or both motors. The level of jitter is not good, but probably not enough to cause mis-steps as long as the step rate is well below the motor's maximum. I then start the other stepper fairly slow (4 per second) and double the speed a few times. This graph is an overview run with one stepper running at about 20 steps per second.