Actually, Wisconsin law says that, if you’re going slower than the “normal speed of traffic”, you must keep right unless overtaking and passing. It also says that if there’s “normal and reasonable” traffic coming up behind you while you’re passing you should increase your speed, up to the posted speed limit.
Wisconsin 346.05(3) states:
Any vehicle proceeding upon a roadway at less than the normal speed of traffic at the time and place and under the conditions then existing shall be driven in the right-hand lane then available for traffic, or as close as practicable to the right-hand edge or curb of the roadway, except when overtaking and passing another vehicle proceeding in the same direction or when preparing for a left turn or U-turn at an intersection or a left turn into a private road or driveway, and except as provided in s. 346.072.
… and 346.59 states:
(1) No person shall drive a motor vehicle at a speed so slow as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or is necessary to comply with the law.
(2) The operator of a vehicle moving at a speed so slow as to impede the normal and reasonable movement of traffic shall, if practicable, yield the roadway to an overtaking vehicle and shall move at a reasonably increased speed or yield the roadway to overtaking vehicles when directed to do so by a traffic officer.
Notice that part 346.59(1) says that impeding traffic is LEGAL if doing so is necessary to comply with the law, and 346.59(2) says you only have to speed up to the “normal and reasonable movement of traffic” if it is reasonable and practicable for you to do so. Also notice that the part about yielding the roadway only applies if a traffic officer directs you to do so.
Now look at 346.57(5): Zoned and posted limits. In addition to complying with the speed restrictions imposed by subs. (2) and (3), no person shall drive a vehicle in excess of any speed limit established pursuant to law by state or local authorities and indicated by official signs.
Notice there’s no exception in there which says you’re allowed to exceed the posted speed limit if it’s the “normal and reasonable movement of traffic”.
So, let’s put these four subs together. Suppose you’re on a road where the posted speed limit is 65 mph and 95% of the traffic is going 75. For the sake of argument, let’s call that “normal and reasonable”. Suppose you are driving a motor home at 62 mph. 346.05(3) says you should be in the right lane. So far so good. Now suppose you come up behind a truck which is going 60 and you want to pass it. 346.05(3) says you don’t have to stay in the right lane if you’re overtaking and passing. You put on your left turn signal and move into the passing lane. At your current speed, you should complete your pass in about 1 minute. But there are cars behind you trying to go 75. 346.59(2) says you have to speed up, if practicable. But by how much? 346.57(5) says you can’t go faster than 65, period, because that’s the posted speed limit. And 346.59(1) says that going 65 in this situation would be legal. So you should speed up to 65 while you are passing the truck. You are not required to speed up to 75; doing so would violate 346.57(5). There is no law that would require you to move to the right lane while you are overtaking and passing the truck. As soon as you complete the pass, 346.05(3) says you must move over to the right lane. At 65 mph, it will take you about 30 seconds to complete the pass.
Notice that nowhere in any of these laws does it give a time limit about how quickly you must overtake the truck and complete your pass. If it takes 10 seconds or 2 minutes, that’s irrelevant as far as the law is concerned.