iPhone wifi speed drops when Bluetooth is ON

I use an iPhone 8 plus - iOS 12.0.1 along with an Apple Watch and AirPods. When bluetooth is ON the wifi speed falls to half. I tried resetting network settings. I did a factory reset. Wifi Networking is turned off. Tried resetting the router. Nothing works. Apple Diagnostics says there is no problem with the software or the hardware of my phone. But the problem still prevails. Anyone with the same issue or with a solution? It's a pain to use an iPhone with slow wifi speeds.