Agree with this. I've been with them for years and over the past 6 months their network speed has crashed hugely in my area (Harrow, Middlesex).Lots of problems with the Three network in general, they've gone downhill over the last year, data speeds in many city centers are just awful and 4G will likely be unstable, they call it congestion.
If you spend a lot of time in one of these dead zones you need to change network, it's not the fault of the phone.
In comparison the speeds my wife gets on her OP3 with EE are nuts - 83mbps downstream and 16mb+ upstream iirc. I get equally good speeds via Three on my 6P but in less towns then I used to this same time last year.