This is a very important point.
Bluetooth interferes with 2.4GHz Wifi, therefore most Macs with bluetooth keyboards/mice struggle a bit on networks at this frequency.
5GHz is pretty much a requirement these days to get the best from a newish Mac.
Typically I would be getting really slow speeds and intermittent disconnects.
Switching to a 5GHz WiFi router solved a lot of problems in the WiFi department for me on both El Capitan and Yosemite.