I have a 2015 Macbook Pro 15", and when I connect the mini DisplayPort to my external monitor, the WiFi speed drops significantly (from > 100 Mbps to < 10 Mbps). I noticed that the WiFi noise level jumps from -95 dBm to -75 dBm after I plug in the DisplayPort cable. I tried several different WiFi channels (1, 6, 11) and there is no significant difference between them, so it's some sort of broadband noise. Since the SNR is so low, the MCS index is set at 0.
The other annoying thing is, once MCS is set to 0, even after I unplug the DisplayPort cable and the noise level drops back to -95 dBm, the MCS is still stuck at 0 and my WiFi speed will not recover. Even turning WiFi off and on again won't help. But this is a secondary issue. My main problem is why display port will introduce such strong noise which kills my WiFi speed in the first place.
The cable I use is a Monoprice mini-DisplayPort to DisplayPort cable.
I got a new mini-DP to DP cable and this time the noise level dropped to the normal level of -95 dBm. However, the MCS is still at 0?? The WiFi speed increased to 20+ Mbps but still not where it should be.