Hi! Thank you both for your help.
I solved this issue 2 days ago, I had a problem in my Windows 10 desktop and I investigated it for a few days till I found this:
I wrote this on my CMD: netsh interface tcp set global autotuninglevel=normal
And it worked!
Now if I run the test on my desktop pc I have more than 30Mbps and before I had 3. I don't know why Windows was configured in that way.