ISPs ain't dum-dums though.
Mine's guaranteed speeds are half of what I'm getting now.
Also, in Italy, the very idea of suing a phone company is totally laughable, and over internet speeds is just not possible.
There is an actual software to test the speed (called "Nemesys" and supplied by the authority that controls telecommunication companies) that has legal value and can be used to break up with your phone company without paying penalties of some sort.
Fact is, I've never been able to make it work.
To start the test, you have to meet a huge amount of bullshit requirements (to avoid some kind of tampering), like: no other program running at the same time, no other device connected to the same network, no Mac, and so on.
Of course, when I booted in safe mode and thrown away all my laptops and phones it usually gave me a nondescript error.
The one time I managed to launch it, I discovered that it needs like 24 hours to perform multiple tests to be sure it's not just a momentary hiccup.
During those 24 hours, if anything goes wrong, the test stops and you have to start over again.
It happened to me, like I-don't-know-how-many hours in, when I got back home from work and forgot to turn off the phone's wi-fi. The phone connected to the network and the test died immediately after.
At that point, I realized I'm totally fine with my speeds.