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.