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The new V2 IO Graph of a capture at the sender can be used to graph the offered windowsize versus the bytes in flight.
You can then easily see whether the slowdown stems from insufficient buffers at the receiver or from a low congestion window at the sender. In this case the green line (bytes in flight) drops when there is indication of packet loss (duplicate ACKs) while the windowsize (yellow) is always above the green line meaning it is not limiting the performance.
The blue line draws the Round Trip Time in milliseconds showing a pretty stable network latency of 131ms.
