Why do my Macbook Pro's WiFi speeds drop significantly when paired with Bluetooth devices? NOTE: This question is NOT a duplicate of Bluetooth and Wifi interfering with one another since Yosemite. That answer pertains to 1) The 2.4 GHz band only and 2) Yosemite only.

Background:

Though currently I’m running Yosemite (10.10.3), I have consistently experienced this issue since purchasing my Macbook Pro (Late 2013, originally running Mavericks). Whether it be a bluetooth mouse, headphones, or even iPhone, whenever I pair a device with my Macbook Pro, my Wifi speeds (running on 5 GHz band) drop by at least %50. I can reproduce this issue repeatedly. It only occurs when a device is paired via bluetooth to the machine. It does not occur when bluetooth is simply enabled on my computer. 

At this point, I just don't connect devices via bluetooth anymore since the WiFi drop is noticeable. I am using a Logitech nano receiver mouse (and using up one of my precious two :( USB ports in the process) rather than using a Bluetooth mouse.

Machine Specs:

Model Name: MacBook Pro (Retina, 15-inch, Late 2013)
Model Identifier:   MacBookPro 11,3
Processor: 2.6 GHz Intel Core i7
Memory: 16 GB 1600 MHz DDR3
System Version: OS X 10.10.3 (14D136)
Kernel Version: Darwin 14.3.0


Speed Tests1:

WiFi speeds with Bluetooth-enabled, no devices paired:



WiFi speeds with Bluetooth-enabled, any device paired:





Troubleshooting:

I’ve ruled out my LAN set-up:


I use a TP-LINK Archer C8 Router connected to a modem provided by my ISP (running in bridge mode). I use the 5 GHz band exclusively. To rule out any band interference, I’ve tested the WiFi speed on the 2.4 GHz band as well. The same drop occurs regardless of the band. 
I’ve used my ISP’s modem as a Gateway to directly connect to the internet (to rule out the TC-Link Router as the cause). Same result. 


I’ve isolated the issue to that of a Wireless one.

There is no speed drop when my machine uses a wired connection (Thunderbolt Ethernet adapter > Ethernet Cable > WAN)

I’ve ruled out a specific Bluetooth device as the cause:

Originally, I thought the issue was with my Logitech M557 mouse. I tested my Magic Mouse instead. Both yielded the same drop in Wifi speed. The same has gone for any device that I paired with my MBP - Apple brand or not. Sidenote: I’ve unpaired the devices after testing. I only have one device paired (though not actively connected). 

I’ve ruled out third party software:

The issue persists without any third party kernel extensions loaded. Additionally, with no other third party software running (Dropbox, for example).  

I’ve ruled out a specific OS X:

This issue has occurred since purchase, in Dec 2013. Across OS X Mavericks - Yosemite. Additionally, the machine has had a clean install of OS X from a fresh Yosemite image. 

I’ve ruled out my Macbook Pro’s hardware:

I ran the Apple Diagnostic Test, repeated times, with no error codes. On a separate issue (blown out builtin speaker recently), I had brought my machine into Apple. I had them run their in-house diagnostic test to confirm no BT related hardware issues. Additionally, as stated, this issue has been a problem from Day 1. 



My Question:

Why does my Macbook Pro's WiFi speed drop by 50+% when paired with any Bluetooth device? Is there a solution?



1Courtesy of speedof.me



UPDATE:

Screenshot of detailed Wifi connection with a BT mouse paired. The SNR and Tx Rate are good:



In fact, nearly identical without BT device paired:
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.