tcp.analysis.bytes_in_flight times lower than communicated window size

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Hi. I noticed that when I'm copying a file over SSH (not SFTP) the communicated TCP window size is ~ 250k, but the actual bytes_in_flight never raise above 50k. What could be the reason? Does Windows ignore this packet field? Is there a tweak for that? The sender is Windows 10 and the receiver is Windows 7.

$ ssh -oCompression=no [email protected] 'dd bs=1M if=/dev/zero' | pv -f >/dev/null
4.53MiB 0:00:27 [ 297KiB/s]

When multiplexing, it's possible to reach the theoretical throughput:

$ ( for ((i=0;i<20;i++)); do ssh -oCompression=no [email protected] 'dd bs=1M if=/dev/zero' & sleep 1; done ) | pv -f >/dev/null
 102MiB 0:01:08 [1.86MiB/s]

The ping is 129ms:

$ ping 192.168.149.55
Pinging 192.168.149.55 with 32 bytes of data:
Reply from 192.168.149.55: bytes=32 time=129ms TTL=126