In the UK, at least, staying in the overtaking lane whilst not overtaking is an offence, and as of a couple years ago can now get you a fine, too. I don’t know if this will be applied if the driver is driving at the speed limit, but I see no reason why it shouldn’t. A simple answer is that an ambulance or police car, travelling legitimately above the speed limit, might need to pass. But just as simple is the fact that the overtaking lane is officially reserved for overtaking. Undertaking is illegal. It could therefore cause issues to other road users regardless of speed. Of course, in the UK, the right hand lane is for overtaking.
And it’s considered speeding no matter how long you’re over the speed limit for. What may have confused the above poster is that speed cameras generally work by measuring speed over a certain distance, like 100 yards. If you brake sharply and go under the speed limit whilst within the distance, it won’t register as speeding. But that doesn’t make it legal, and if a policeman using a radar gun catches you speeding, it doesn’t matter if it’s only for a second. What is true, again, in the UK at least, is that they allow a certain amount of leeway, usually a couple of miles an hour, before they give a fine. It doesn’t make it legal, the guidelines are just flexible. They’re actually planning on changing that soon, so that drivers are penalized even at 1mph over the limit.