Baseline using fq_codel and 54.5Mbps downstream limit and 10Mbps upstream limit:
Now, turning off the downstream shaper yields:
This indicates (to me), that there's very excessive buffering at the CMTS, and the downstream shaper is badly needed to keep it under control.
So I re-enabled downstream, and disabled the upstream shaper:
Again, not ideal. It's interesting seeing how the shapers interact with the traffic in the other direction as well. Because their ACKs are going through the other path, disabling shaping on one clearly effects the other.