> I suspect it's the backhaul from the mobile mast but I'm not sure.
It might be in some cases, but in most cases that backhaul will be a very scalable fibre line. It's probably more down to limited spectrum. Three attracts heavy users, because their pricing is designed around them, so they have the most loaded network, and they can't just manufacture new spectrum (they can add new towers up to a point, but there's a lot of cost to that). 5G should help, somewhat.
It might be in some cases, but in most cases that backhaul will be a very scalable fibre line. It's probably more down to limited spectrum. Three attracts heavy users, because their pricing is designed around them, so they have the most loaded network, and they can't just manufacture new spectrum (they can add new towers up to a point, but there's a lot of cost to that). 5G should help, somewhat.