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With something like RED or WRED working on TCP flows, I guess you get to see random packet drops kick in earlier than "total congestion", aimed at slowing each TCP stream. You wouldn't see this on the ping graphs at all, but I imagine the kind of single-threaded slowdown visible on your TBB speedtest would be the best indication.
IIRC, "old-style" congestion would affect both the single-threaded tests and the multi-threaded-tests. I imagine that signs like your graph (very slow single-threaded, but relatively healthy multi-threaded) are a sign that some form of RED/WRED is in place.
The symptoms are commensurate with simple dropping of packets. Packet loss is capping each TCP flow traversing the choke point at 8-10Mb or whatever, hence the 6 x TCP flow test will show a significantly better result than the single flow.
This isn't fancy queuing it's relying simply on TCP's own congestion control. It's a reason why P2P has been so problematic on networks in the past, especially before the creation of UTP, and why Usenet continues to be. Loss being caused by rate limiting on a port will cause a per-TCP flow limit and those applications, being multi-threaded, will crowd out single flow applications such as streaming.
RED is, actually, more likely to bring single-thread and multi-thread results closer together.