I've put onto github ( https://github.com/colinpaicemq/TCPIPMonitor  ) a program to display stats about TCP interfaces, and stats about a port - such as the buffer sizes.

With the port monitor, it will display when the receive window etc changes, so you can see if there is a change in throughput when you detect a slow down
I've also blogged about TCPIP buffer usage (https://colinpaice.blog/2024/04/27/one-minute-networking-tcp-buffer-sizes/) to help you understand.

These use the C NWMServices available in TCPIP so you can see how to use them.

Thank you Colin. You have been busy !.  I have been busy with a machine upgrade recently but I now have the time to try this.