It’s definitely a hazard. I had a situation today where there was a truck ahead, but behind the truck was a car. Turned out the car was going way slower than the truck, I wasn’t expecting the car to be going so slow at all.

And, more generally, if someone isn't driving at the speed limit, it can force others into riskier overtaking. I never felt safe being driven around by my granddad.

I’ve heard that optimal fuel efficiency is about 65mph so it might explain why some people drive slower

All my granddad's car wisdom dated back to the 30s and 40s, so he didn't get much above 40mph.