Bluetooth causing extremely slow wifi

So I've had my m8 since launch, about May of 2014. I've never once had a problem with my wifi, until last week. All of a sudden, I noticed all of my wifi connections (home, work, etc) are all extremely slow. Speed tests are barely getting 1mbps down or up, and the test itself is very erratic, bobbing up and down the whole time. All my other devices are fine, and nothing with any of my wifi connections has changed. I uninstalled about 3 apps that I had installed within the last month, with no change. Verizon sent me a replacement m8, and I got it yesterday, loaded up all my stuff, and low and behold, still slow wifi.

After talking with numerous Verizon techs, they can't solve it, even after multiple factory resets. However, this morning I saw a post while googling it, not an m8 but still android. They said that if Bluetooth is on, but not connected to a device, it could cause issues. So I turned my bluetooth off, and speeds were perfect, getting exactly what my internet connection is. Turned it back on, and back to 1mbps.

So one, why is this happening? I've read that the bluetooth and wifi antennas are the same, and that could cause interference, but it should not be this bad. Second, why did this just now all of a sudden start? I didn't usually leave bluetooth on, but I got a new car about 3 months ago, and decided to leave it on to automatically connect when I'm driving. But it was fine until last week, why the change?