This has been a known issue for it seems quite some time now. I use a K760 Logitech bluetooth keyboard with my mid 2014 rMBP and noticed that with the bluetooth and wifi on at the same time my speeds dip to as low as around 5mbps. Without it on, I can get at worst 15mbps. Usually my speeds are 25mbps without the bluetooth on and 15mpbs with the bluetooth on which I can for the most part deal with. However, when speeds dip, the internet becomes unusable with the bluetooth on. I've researched a lot of "fixes" but nothing really seems definitive. Are users like me just waiting it out for a software update? This seems ridiculous.