When you’re troubleshooting network performance (or rather: the lack of it) you might find the following tool useful.
It allows you to set up a test server (NetCPS -s) which you can then use to be contacted by several clients (NetCPS [server IP address or name]).
It doesn’t require installation and source code is provided as well, in case you might want to customize/tweak it a little or compile it to support other platforms than Microsoft Windows.

Beware: Governmental and military use is NOT permitted by the author.

NetCPS: Freeware network throughput performance test for Windows