“Fast Car”

In one of your lessons you used the “fast car” as an example referencing the quest to find one’s fastest most efficient picking motion. And the need to stick to your fastest movement because you can always slow it down to help gain control and train the left fretting hand to slowly work up to the faster speeds. That seems to be where I’m at now. My fastest movement is from the elbow joint as is probably obvious in the video… so the challenge is slowing that movement down while preventing it from morphing into an entirely different movement. It’s very difficult to do while keeping the same exact movement. Easy to do it fast on just one string, but also very difficult to maintain the same movement over multiple strings. Any suggestions on how to establish a slower speed, maintaining the same form, so I can now focus on the left hand?