I have been experiencing the same issue as well. Not sure if this is hardware or software related. Although a nuisance at this point, I’ve gotten use to turning off the Bluetooth (from control panel) whenever I don’t need it. At least for me, when Bluetooth is on (New Bluetooth Connections is on) the internet speeds over wifi are unbearable to the point it becomes useless.
iPhone X
iOS 12.1
Hoping this is software related and can be fixed with a software update.