Mid 2015 Mbp wifi issue by location
This is not the perfect answer, but it is an answer...

Move everything away from your MBP that's plugged in to it.

When in location A my set up is different to a more cramped set up B.

My MBP was sat underneath a 43 inch ultra-widescreen monitor, with a usb 3.0 hub containing, amongst other things, my external drive, sat atop it. As soon as I separated them all as far away from each other as possible the speed went back up. Not back up to anything like it was with nothing attached, but acceptable.

Hopefully might help someone in the future. No idea why it's even an issue, I'm guessing everything emits something that clashes (potentially) with something else.