Lines are the steady-state median of each run. The shaded band is the 10th-90th percentile within that run — a wide band means the rate was swinging, not merely low. Download collapsing while upload holds is the signature of this fault.
Idle versus during the download. A large gap is bufferbloat — queues filling somewhere upstream, which makes a line feel slow even when the throughput number looks acceptable.
Hop 1 is the UDM Pro. Hop 2 is M-net. If hop 1 stays flat while hop 2 swings, the problem is not on your side of the line.