I finished routing the DAC voltage to all 9 boards, and decreased the resistance of the pull-up on the LM339 used as a voltage follower to 1K. I've increased the sensing sweep to 16 steps (but the first step is always 0V). I also updated the serial interface on both ends to ship out the touch level data, which involved removing all the unpacking code. I've captured the analog sweep on the scope with and without the serial connection active.

Without serial data: 53Hz internal refresh rate:

And with serial activity and a 49 Hz refresh rate.

So as long as the processing sketch is set to less than 45 as a framerate, it should work just fine.

I'm still not sure why it just worked, but I didn't have the problems this time around with the serial data from the Teensy to processing being slow or otherwise annoying. I did another quick video showing the full touch sensing with the depth capability.

The hardware reports the state of the touch system to the processing sketch, which decides what color to set each cell to, and sends that back to the Teensy. In this case I did a simple linear scaling of the touch distance to grayscale. In the video, it's easier to see the gradients in the processing status window.