I realize the original post is from a person in Italy, and the law and the etiquette may be different there. Most people in the USA don't seem to realize that most state motor vehicle codes REQUIRE you to move right if you are slower than other vehicles regardless of the speed limit. The Texas "Transportation Code" spells this out in very obscure legalese as follows:
Sec. 545.051. DRIVING ON RIGHT SIDE OF ROADWAY.
...big long section (a) skipped...
(b) An operator of a vehicle on a roadway moving more slowly than the normal speed of other vehicles at the time and place under the existing conditions shall drive in the right-hand lane available for vehicles, or as close as practicable to the right-hand curb or edge of the roadway, unless the operator is:
(1) passing another vehicle; or
(2) preparing for a left turn at an intersection or into a private road or driveway.
The phrasing here is very awkward, because paragraph (b) is a sub-section of the whole "driving on the right" part of the code. Take note that it does NOT say anything at all about speed limits. Instead, it goes to great pains to state "[if you are]...moving more slowly..." than the "...normal speed of other vehicles at the time and place...". This has been tested in court many times and it can be stated another way quite simply: If approached from the rear, move right. No matter how fast/slow you are going, the car behind you is the "other vehicle" at that "time and place".
Most states have similar laws.
In my entire driving life, I've noticed a broad section of people who seem to feel that if they are doing the exact speed limit, they can stay in the left lane and 'block' cars that want to go faster. (Clearly, the OP was not doing this.) I've always found it fascinating to watch these people creating a traffic hazard by being slower than the general pace of traffic and insisting on staying in the left lane. I imagine the mental state of these blockers to be: "I'm doing the right thing, I'm doing the speed limit, these other jerks can just piss off" or similar. Fascinating to know, for a fact, that these blockers are actually the ones breaking the law.
Anyway, the answer to the OP's question: MOVE OVER!! It is the safe thing to do, and it is probably the law as well.