I'm experiencing the same issue, iPhone 6 16GB has been replaced already because of that. Nevertheless, none of the solution listed worked. It works then it doesn't.
Very easy way to test your WIFI speed is with an app called WIFI SWEETSPOTS.
This app tests the speed of your wifi connection and not your Internet speed, very useful for this problem.
Went to Apple for this problem and they act like idiots because of course they don't know what to respond when you demonstrate the problem in front of them with the very useful WIFI SweetSpots app.
Apple probably knows about the issue, but as usual they are really good at pretending it dos not exist.
I have tested this problem on many different routers, I own an ASUS RT-AC68U, wifi speed test goes as high as 280 Mbps (more or less) when the iphone 6 is not acting out. And like most of you it would drop down like crazy for no reason. Of course a restart of the phone fixes things but it comes back. I tried different routers, 2.4 Ghz, 5 Ghz, WEP, WPA, WPA2 anything you can thing of as well as resetting the phone as new and other crazy things we are suppose to do to make this $700 phone work properly. NOTHING AS WORKED SO FAR.
Cheers.