My router which had previously operated on channel 1 switched to channel 11. This was accompanied by a drop in speed when tested on the wifi connection speed from 37.x Mbps to around 2.x Mbs. LAN speed remained a consistent 37 or so Mbps. Channel 1 not congested, tested using a wifi analyzer app. Reset the router this morning and now back on channel 1 and wifi speed consistent with the LAN connection speed. Why did the channel change? Why should the speed drop? No idea. May be helpful if anyone else has similar problem.
Possibly your router detected it has become congested, some routers have the ability to choose the best channel.
Why should the speed drop?
Channel 11 might have been better than channel 1 but still worse than channel 1 was originally. Perhaps there was some transient broad spectrum interference. Perhaps your neighbours have been changing channels and caught your router out. Sadly it's just another case of wifi being a bit ropey. If you want reliability go wired.