Do you have any Sonos devices or devices that create their own mesh (e.g. Amazon Echo, Sky Q) connected to the BT Smarthub/Complete Wifi system? I'm wondering if the Complete WiFi system doesn't work well with some of them.
I've got the SmartHub 2 and two black Complete Wifi discs and after initially being pleased with the total wifi coverage I was getting, I noticed the speeds were very variable, mostly slow, around the house.
After seeing this thread it prompted me to dig deeper: https://community.bt.com/t5/BT-Devices/Whole-Home-WiFi-Sonos-problem/m-p/1910452#M7329
I've found that since connecting one of the Sonos speakers to the router via ethernet cable the speeds are very stable and I typically see 68-72Mbps on all my devices (I have an 80 down/20 up connection).