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This looks like a buffer bloat issue. Data is sent on a high speed network and needs to be forwarded onto a WAN connection with lower bandwidth, this results in buffering. But when the buffer fills up, packets will be discarded. Once the packet-loss is detected, the sending side will reduce it's congestion window and this means less data can be sent per roud-trip time. This results in reduced bandwidth.
In the slow transfer, the max window size seen is ~16MB, this is probably larger than the buffer on the WAN router, which is the why the buffer bloat occurs.
In the fast transfer, the max window size seen is ~256KB, perhaps this was preventing the sending system to overload the buffer.
It could also be that there is different equipment with different buffer sizes on both locations, causing different behavior.
You could try to limit the maximum window size on the receiving system of the slow response. See if it makes a difference.
