The highway speed scatterpot in the US has a floor at the "speed limit" and the median speed is usually 10mph to 15mph higher than that. In this scenario, driving the "speed limit" means nearly all other users will overtake you at a significant differential, and some in a disorderly manner. Consistently doing so in heavy traffic provokes backups and the usual consequences to other drivers.
My experience on US highways is that it generally isn't quite that bad: if you're going roughly the speed limit, you will be in company with a significant subset of the commercial vehicles, whose owners mandate following the speed limit (and in some cases enforce it with GPS surveillance).
Your description sounds right but also pretty similar to GP’s characterization of a floor at the limit and 10-15 mph higher median with the result that more than half of traffic is passing you at a 20+% speed differential.
