iOS 11 iPhone WIFI slowness

Hi

Ever since I updated my iPhone 7 plus to iOS 11 the wifi performance is shocking and it seems to be just effecting the iPhone as if I run a side by side speed comparison with my iPad ( same version of iOS ) there is a ten fold difference in download speed at best and sometimes the iPhone has a reported download speed of 0.1 MB/s

Its not a hardware issue as far as I know as I have now moved to a new iPhone with a SIM card in and I see the same issue which on most occasions makes the iPhone totally useless when on Wifi

I have noticed there are several people are having similar issues and I have tried various things including

Changing the router

Switching from a 2.4 GHZ wifi SSID to a 5GHZ wifi SSID

Hard reset of the device

Reset of network settings

Switching off mobile data and just having WIFI

None of which have made a difference so if anyone has any ideas I would be grateful as its driving me nuts

I had hoped the latest iOS 11 update would help but it hasn't

Thanks in advance

Simon