The image appears to be from the UK. Here in the UK cyclists are supposed to stay at least 0.5m from the kerb, with a recommendation for more distance if possible (rule 72 of the Highway Code). Cars are supposed to keep at least 1.5m away from cyclists when overtaking (rule 163). Taking an average cyclist width of 60cm (some handlebars go much wider than that, as might pannier bags, but let's use that as an average), that means a single cyclist should have control of ~2.6m of the lane at least.

Let's say that the average lane on urban roads in the UK are around 3m wide (an estimate based on a quick google, not a rule), this means a legal overtake of a cyclist should have the car leaving no more than 40cm of the car in the lane. It's not a big jump from that to moving entirely into the other lane.

Admittedly almost no one in the UK actually follows these rules, but this is how it's supposed to be. Given that, adding another cyclist riding abreast shouldn't affect overtaking time significantly, whereas the two cyclists riding in line will double the amount of time in the oncoming lane.