The rule for the leftmost lane (highway) is that you must not block for other drivers. It is in the rule book (at least in my country). That mean in very clear terms that if you can't do the overtaking in a timely fashion without blocking other drivers, then you should not enter the left lane.

If there is one thing that tend to cause conflict and trigger dangerous situations in traffic, it is when someone driving at 0.001% faster than the next car enter left lane while maintaining the exact same speed, basically matching the speed on the right. That is just as illegal as speeding.