Exponentially both ways though I would argue, often, the slower driver is more of a hazard to other drivers!

If someone burns past in the left lane unless someone else does something wrong (like move lanes without looking first) or causes rapid traffic slowdown in the left lane either by merging poorly or being too aggressive on the brakes, they are more or less not a risk.

If someone is driving too slow they are dangerous without anyone else making a mistake - if you or anyone behind you doesn't have visibility (eg behind a truck, around a bend, glare from the sun etc) then there's a hard braking event, which is always dangerous. The more slowly compared to prevailing traffic they go, the more attentive other drivers need to be, the more dangerous it becomes.

No, the "speeding" part is from my own experience. The speed limit here in Wisconsin is 70MPH, but hardly anybody follows it. In any case, a driver who cannot handle a 20-25MPH speed differential should not be on the road, period. Most people can't even handle going that speed on residential streets, it's that slow. I propose the following thought experiment to make it obvious:

Two identical highways, both empty. On one, the U-Haul driver following the company's 55MPH speed limit. (Minimum speed on 70MPH highways is often 45MPH, by the way.) On the other highway, a driver following the speed limit at 70MPH, but is who cannot handle closing in on things at 15MPH. Distraction, eyesight problems, who knows? The slow, 55MPH driver will have no problems. The faster driver has a high likelihood of crashing into things, even without slow drivers in the way.

If speed differentials are that much of a problem, then it's the faster drivers that are the source of it, and the speed limit is too high.