What does happen in that situation sometimes is that if you're speeding along with everyone else, that sometimes gets allowed as a defense against a speeding ticket; basically the argument is that if driving the speed limit when all surrounding traffic is driving over the speed limit, that the relative speed difference can matter. At least that was true when I learned to drive (many, many years ago).

Right. What's (maybe) illegal (and definitely unsafe) is staying in the left lane, although driving at the posted speed limit, and impeding traffic that wants to pass.