Bluetooth and WiFi interfering with one another since Yosemite
Bluetooth and IEEE 802.11/b/g/n (WIFI) use almost the same frequency bands:


Bluetooth: 2.402 - 2.480 GHz (79 channels)  
WIFI 2.4 GHz (IEEE 802.11/b/g/n): 2.4 – 2.4835 GHz (11, 13 or 14 channels depending on the country)


Usually Bluetooth uses frequency hopping and changes the possible 79 x 1 MHz bands 1600 times a second to avoid disturbances while WIFI uses fixed channels.

Obviously something went wrong developing the latest Bluetooth- or WIFI-drivers.

The only workaround until Apple gets their drivers fixed is using dual band wireless access points. Those devices use two different frequency bands:
2.4 – 2.4835 GHz and 5.18 - 5.825 GHz (with some gaps).
Connecting to WIFI 5 GHz (IEEE 802.11a/h/n) will not interfere with Bluetooth.

Almost all Macs sold since 2006 support 802.11 a/b/g/n.

Apple's answer: Wi-Fi and Bluetooth: Potential sources of wireless interference

BTW: Not only typical RF-based devices (like wireless phones, etc) but also bad-shielded USB3-devices and cables interfere with WIFI (2.4 GHz) and Bluetooth.