Why do my Macbook Pro's WiFi speeds drop significantly when paired with Bluetooth devices?
I found this answer here: https://discussions.apple.com/thread/6803542, a guy named 'Todd Berliner' had similar issues. Credits for him if this fixes your issue

Todd has similar problems and worked with an Apple senior supervisor for several weeks and came to the following fix:


Turn off bluetooth.
Disconnect all peripherals from your computer.
Open System Preferences. In the "Energy Saver" preference, de-select "Wake for network access" (no check, and leave it that way).
Open the finder. Select the "Go" menu.  Select "computer." Select "Macintosh HD."  Go the the "Library" folder.  Go to the "Preferences" folder. Find the preference file "com.apple.bluetooth.plist."
Rename "com.apple.bluetooth.plist" as "com.apple.bluetooth.plist.old" (i.e. add ".old").  Eventually, if this solution works for you, you can delete this "old" preference file.
Restart your computer (this will create a new "com.apple.bluetooth.plist" preference file).
Turn bluetooth back on.


Apparently, the problem is a bug caused by a bad interaction between the bluetooth preference file and wifi.