Would you say "don't get worked up and leave 10 minutes earlier" if someone decided to drive at 30mph in an NSL? What about if they were doing 20mph?
The issue is that often people that are doing inappropriately slow speeds are doing it because they haven't got the ability to go higher speeds, whether this be due to their age, eyesight, tiredness, intoxication, they get easily flustered or condition of their car is bad - so you then have to question why should we put up with it? If they don't think they don't have the reactionary ability to go more than 40mph on a clear NSL road, how are they going to fair if something pulls out in front of them or a child runs out? I have said it before, but I can see it in my own dad. He used to drive at appropriate speeds etc, but in the last view years, when i've been in the car with him, he'll slow down to 30mph for some reason on a bend which you can quite clearly take at 60mph. And again, he'll sometimes do 40mph, whilst I'm wondering why he is going this slowly. It's because he's getting older and his reactions aren't as good.
I regularly pull over people who are doing inappropriately slow speeds and 90% of the time they are old, have poor eyesight and are just "not really with it". Would I trust them to take avoiding action in an emergency? Would I hell.
If it is safe to do 60mph, then you should be doing 60mph in an NSL. If it is safe to do 40mph in an 40mph then you should. If you are doing less than that, why? Doing inappropriately low speeds causes people behind to take unnecessary risks, and causes whole trains of cars with people not keeping safe distances - all because one person at the front of the queue wants to do 20mph under the speed limit.