"narrow" roads ?
Rather difficult to define for me round here. (Cheshire)
Some are so narrow that a cyclist would have to pull in to a gateway to let a car past.

Overtaking a short group of cyclists is always going to be easier and safer than overtaking a long stream of them.
I was taught to overtake (anything) using the full width of the road. Two three or four... abreast would be fine to me .... depending on the width of the road.

Even slight inclines mean that a cyclist's speed can vary considerably, faster or slower. In a car it is quite difficult to judge that.

There IS something frustrating about being behind a slower vehicle/bike/sheep and it can be stressful making an overtaking manoeuvre.
Timewise could someone do the maths on how long the delays might actually be when we slow down before overtaking?