In this case speed test or any other internet web site tool was not useful.
Solution was using Iperf in command line mode launching a server session in remote site and a client session on another site.
In this way there was possibility to stress network and measure relative performances.
Here they are some video examples:
http://www.iperfwindows.com/screencasts.html
Meanwhile I am taking note about a good website dedicated to test internet performances.
[update 2022.08.31]