Slow wifi iPhone & iPad. BT Home Hub5

We keep experiencing slow wifi speeds. Our ISP is BT, we are using a BT home hub 5, when we reset it we get latency 19m/s, download 53mbps, upload 9.5mbps but after a couple of days we have latency 80m/s, download 2mbps, upload 4.5mbps these checks are done using the speedsmart app, which is installed on all our devices 2 iPhone 6s, iPad Air & iPad Air 2. If we check the broadband speed using the BT app we are getting good speeds to the hub itself. BT engineers have renewed our line to the property, replaced our hub box and are happy there device is wor correctl, we have tried switching channels on the hub itself but still no good. My question, is anyone else in the UK experienced this problem? Is there a fault on the iOS devices with the wifi, compatibility etc.