I wonder if this was some weird coupling/interference between wireless and the actual modem/NIC in the path to the internet? Bouncing off a separate machine would've desynced the two streams just enough that the wireless interference doesn't clash with the outbound NIC (or the other way around - the outbound NIC of the modem was causing interference which was slowing down your wireless).
The router was plugged into a Cat6 cable which ran to a switch in a closet in the building’s networking room, which was connected by fiber to the network.
No modem within 15 floors.
Interesting guess though, wouldn’t have thought of that.
