Pokey McSlowpoke is not the problem in your first example. The danger is caused by all the Ragey McTantrums and their lack of self-control.
With a speed limit of 40, most people are driving 45 to 50, so the faster driver is 5 to 10 MPH away the average, and the slower driver is 20 to 25 MPH away from the average. Plus, everyone has to react to the slower driver, including people who aren’t paying that much attention. With the faster driver, he’s presumably paying attention, and going around others. So the slower driver is worse.
If you had a situation where both were plowing/plodding along in their lane, oblivious to other traffic and making others avoid them, then the faster driver would be worse. But I’m assuming that’s not the case here.
It depends in part on the nature of the road.
In my area, a speed limit of 40 means that you’re on a main arterial that is not limited-access. There will be cross streets and driveways in and out of businesses, and there will be traffic lights at major intersections. In that case, the faster driver is more dangerous. The other drivers on the road are prepared to slow and stop if necessary, so the slower driver is merely annoying.
If we were talking about a limited-access road where drivers had a reasonable expectation that everyone would be going about the same speed, perhaps the slower driver would be the greater danger, but limited-access roads usually have higher speed limits.
Slow drivers are more annoying
Fast drivers are more dangerous.
A slow driver you can easily see and avoid. A fast driver you can see and avoid (in your rear-view mirrors) but it’s much more difficult to do so reliably.
I don’t understand what either of you is saying about “turns.” Are you guys thinking of mountain roads or something?
To me a “turn” is something you should be stopping (or slowing way down) for anyway.
Significantly below.
A driver or rider travelling way over the limit isn’t going to bother you long. Either he just roars past you in a lane to your left, or she finds a good place to overtake over the dotted yellow, or you just move over to the shoulder to let the car pass. You hardly have to do anything.
A driver travelling way under the limit forces not just you but everyone else to try to find a way around. Traffic clumps up. Then you’ve got the choice between passing each car in turn to get to the slow car so you can pass, or cranking the throttle all the way open and putting your belly on the tank to get past everyone at once.
Above is worse, because not only do they endanger others by being at a significantly different speed, their control is going to be worse. At least someone going too slow isn’t likely to lose control and run over a sidewalk full of people or into a house.
Maybe. A lot of the people I see going way below the speed limit seem to be doing so because they’re too distracted to realize that they’re going so slow.
Both are dangerous but I would give a slight nod to the speeder.
Speeding is much more common and more likely to result in a serious injury and/or fatality. Also, you can probably see the person going slow beforehand whereas the speeder is going to come up on you very quickly.
Faster because when you hit the impact is going to be that much worse.
And it’s “brake”, people. Not “break”. Put it this way: I will “break” your heads if you don’t use “brake” properly.
Hmm, Anaamika, why not just give 'em a brake. 
Though you’ve stated 40 mph, I prefer to visualize this on the highway, so adjust everything for an average limit of 55 mph.
For me, the person going 25 below is the bigger problem. Cars typically have to go around them, causing people to brake, change lanes at the same time, and other annoyances/road rage. It’s typically a point of frustration, where people will then pass the person at 10-15 mph+, anyway. To add to this, slower drivers can be indifferent, distracted, over-reactive, or less reactive, depending on why they are moving that much slower.
For the most part, if someone is doing 10-15 over, I can see them approaching in advance (and get over to the right, if I’m in the left lane), and they’ll usually pass and be out of my way as they continue into the distance. In all honesty, I consider an actual attempt to pass someone at 10-15 to be normal, not necessarily speeding, and at least with traffic laws in my city, there is a 10 mph buffer, anyway (speed cameras). Either way, faster drivers can also be indifferent or distracted, but they more-so try to get somewhere quicker, so they do seem to be attentive and quicker to react. Poor judgement is the bigger problem here, though. They also serve as a nice line of defense for state troopers.
More critical, is how aggressively the faster car is changing lanes. That’s when speed becomes a problem, IMO, because it’s less predictable. Otherwise, it’s not uncommon for people to do 25 over in a 55, here; 15 is absolutely workable and you’re certainly not blowing by cars at that rate. Humans can sprint faster than 15 mph on foot, naturally.
Except that unless they’re experiencing a severe breakdown, no one drives 30 mph on a 55 mph-limit highway.
If we’re talking highway driving, I view the person weaving in and out of traffic doing 80 mph as a hell of a lot more dangerous than someone doing 50-55.