If the traffic flow is usually 40 mph during commuting hours, it’s not because of “unnecessarily slow drivers.” It’s because traffic slows down as the road gets more congested. Traffic slows down long before it gets to “bumper-to-bumper” level - it’s a gradual progression. Read up on the fundamental diagram of traffic flow.
It is not the same as suddenly having two weeks off, but there is definitely an impact. Back in the good old days traffic in the Bay Area was reduced during the summer. That let me get home to dinner earlier, read the paper without rushing, and maybe do something on my list. Before I retired, as our traffic got worse and worse, I felt more and more stressed. Hell, the traffic was a significant reason why I retired early.
When you can actually see a clear road ahead and you’re sitting fourth in a row of cars behind someone doing 40 mph on a 60 mph road, then yes, it is someone driving unneccessarily slowly.
Because when/if you get past you can drive quite easily at 60 mph and watch them recede into the distance behind you.
I’m not talking about city traffic, thankfully I work on the outskirts and don’t have to do that anymore.
It may look like “clear road” between the groups of cars. But the very fact that you are running into these “slow drivers” all the time, such that your average speed is 40 mph (which is your claim), indicates that the amount of traffic is what’s slowing you down, not just a few isolated drivers.
I kind of had a commute like that at one period in my life. Narrow, single lane road, lots of curves, no shoulder, you couldn’t see very far ahead. And for some bizarre reason bikers liked to use this road despite the fact that it seemed to me any biker using it had a death wish. I routinely got stuck behind a biker, not every day, but several days a week. It was completely unsafe to try to pass. My normal 15 minute commute would turn into a 35 minute commute whenever this happened. I developed an extreme hatred of bikers for awhile. Actually, I still am not very fond of them.
It seems to me that the last thing you want to say to a slow driver is “Stop it!” 
Maybe those bikers lived and worked in the same general areas you did, and therefore ended up using the same road for commuting.
Oh, but the math is still problematic. If there was a “21-mile, continuous segment of a single-lane, 60-mph road”, the OP wouldn’t be able to complete his commute in 30 minutes while staying under the speed limit, unless the 21 mile stretch was essentially his entire commute. If this was the case, his rant would have been about how these inconsiderate bozos are going unnecessarily slow for nearly the entire commute, not on a single segment.
No, as frustrating as it is, the one or two slow drivers are not the cause of the increase in commute time. As has been already stated, the cause is TRAFFIC. The problem isn’t with one or two slow drivers, it is with all the drivers. The slow ones, the ones who want to go fast, but can’t go fast safely, and the ones who go faster than is safe.
Yeah, OP, you are part of the problem, along with everyone else.
Or they simply have no idea where they’re going, and apparently have no concept of how street and building numbers work. These are the ones you get behind, and they’re clearly looking for a particular place to make a turn, because they keep braking at every intersection, even when there is no traffic signal or stop sign, and then speeding back up to a speed that is still 5-10 miles below the limit, only to repeat at the next intersection, and the next … It doesn’t occur to them that if they miss their turn, they can circle the block and come around again, so they slow way down for everything that looks like it might possibly be where then need to turn.
The OP would like this link about a 59-year old man in Minnesota who lost his license after repeatedly given warnings for driving too slowly. He didn’t see any problem with driving 30 MPH in a 55 MPH zone, just in case some “critters” cross the road. I wish they would do this more often.
Yes and no, often when you can get past one slow driver you have quite a bit of clear road until you hit another one. It isn’t all the time, it only takes a handful or less of slow drivers to jam the whole system up.
I have a sat-nav in the car, it gives an estimated time of arrival and you can watch it increasing or decreasing depending on whether or not you are stuck behind someone.
I’m easily amused.
Actually if they pulled over and let everyone past I would be quite happy 
That’s certainly the case with a large number of cars trying to get into a small area like those going into a city, its not the same as on a single lane road where the physical volume of cars isn’t particularly high, even during rush hour.
By law slow cars are supposed to pull over every so often and let faster vehicles past, of course nobody actually does that.
Thanks, I also liked reading about the 90 something year old man in the US who gave up his licence because he was tired of all the goddamn slow drivers 