While a speed limit is not a target, you should still be able to keep up reasonably with the flow of traffic and not be unduly hesitant or hinder other drivers. In the driving test, if the speed limit is 60, and the conditions (weather, traffic etc) do not prevent you from doing the speed limit, you are expected to demonstrate that you can competently control the vehicle at that speed on that road. If you are doing 30 or 40 on a road with a 60 mph speed limit and there is no good reason for going that slow, then the examiner can and probably will mark it down as a fault - or even fail you -because he/she will assume that you don't know what the correct speed is for that road, and may also assume that you cannot control the vehicle and are too hesitant. If your speed is so slow as to cause a tailback, then that's a problem.
There are many people who drive recklessly but that is not always down to driving too fast; it's incompetent driving. I've seen plenty dangerous situations caused by drivers who have been driving too slow and have been completely oblivious to what is going on around them. Driving slower doesn't always mean safer. You have to be completely aware of what's going on around you, from the speed limit of the road you are on, the volume of traffic, how it's flowing, what's ahead, what's behind, the weather conditions, whether it's daylight, dark... etc. Assuming that you are safe just because you are driving slower is just as dangerous as driving too fast. Bear in mind you can get stopped by the police and penalised for going too slow as well as too fast.
My partner and I are both petrolheads; we love cars and we love driving. He owns a supercar and has extensive experience of hill-climb racing, which requires exceptional driving skill. He can drive at speed but he is an extremely safe, competent and responsible driver, and I feel much safer in any car he drives then I ever do in my mother's car (who drives very slow and hesitantly); in fact I refuse to get in her car now.