I regularly drive from the end of the A11 to the N Norfolk coast, a long stretch of A roads and small roads, and I do understand your frustration. Swaffham to the coast behind a lorry or caravan or 40mph-er is tedious and there are v few places where you can safely overtake.
However, frustration is the problem and the potential danger. I remind myself that people have a right to drive at the speed they feel safe at, that many people are simply not familiar with the roads and their bends, that there really is likely to be a hazard round every bend - bike, tractor, horse or whatever, and I have no more right to dictate the speed than any other road user.
I am not a slow driver, but there are lots of stretches of road where I wouldn't be doing 60, and yet other drivers make a risky move to overtake at 70 or faster. I daresay they are fed up with me.
Wet or windy conditions, dark or dusk, all reasons to slow down.
It can be very frustrating, but all we can do is take a deep breath and keep calm and keep safe.