I fully expect to get flamed for this because I know I'm a tad over the top about this but it's my pet hate so here goes.
If say be gentle but I know this is the wrong place for that 
I drive a lot. Every day I drive on a road that's technically a country road as there are fields either side but it's more than wide enough and not too windy. This road is followed by two straight roads. These are all national speed limit roads.
This is comfortably attainable with a few corners on the country road where 50 is a better speed. Obviously various weathers call for different speeds. I was behind a double decker bus doing 50 down there today.
The last few days I have been stuck behind people doing 35-40. Today the lady flashed at me when I finally get chance to overtake.
They way I see it is if you are driving so slow on the roads either
A) you are unaware of the speed limit/unable to follow basic road signs and therefore shouldn't be driving
B) do not feel comfortable driving at an appropriate limit for the road and therefore shouldn't be driving.
I know it's a limit not a target but you fail your driving test if you 'fail to progress' so they shouldn't be driving like this.
It's the principle that they don't care about anyone's time frame or how they affect other people. They just trundle along deciding what speed they want to go at and not giving a fuck.
We all have bad days where we mess up roundabouts and misjudge but Jesus Christ, if you can't drive at 60 in a straight line...