USB3 may be ruining your Wifi speed
How having a second monitor may be degrading your wireless network.
I settled down to work one day and found my internet connection painfully slow. I fired up https://fast.com to measure the speed and it was somewhere between 0.5Mbps and 3Mbps.
Later in the day, I found things had improved greatly — I was measuring 25Mbps, which is about as fast as my ADSL connection will go.
What had changed? I had unplugged my computer from its second monitor!
Can having a second monitor degrade your Wifi connection?
Surely plugging in a second monitor couldn’t make a difference to your network speed. I set about measuring internet speeds with and without the second monitor connected:
Sure enough, network speeds were much, much faster without the USB3-HDMI adapter plugged in.
What could possibly be going on?
USB3 and 2.4GHz Wifi don’t play well together
A tip from a colleague alerted me to the fact that some USB3 chipsets can emit electromagnetic interference which can deteriorate 2.4GHz Wifi signals.
I used the Mac’s Wireless Diagnostics tool to record signal quality over time as I disconnected and reconnected the USB3 cable.
Solving the problem
My Wifi router was set up to have one network id on two frequencies: 2.4GHz & 5GHz. My Mac had chosen to use the 2.4GHz variant and couldn’t be persuaded to switch to the other frequency.
In the end I split the networks to have two different names so that I could force the Mac to use the 5GHz network.
The signal quality is now unaffected by the second monitor connection and the network speed is a healthy 27Mbps.
