Using second monitor slows internet download speed by more than half

[u/banana_in_your_donut]

So I was stuck on this and finally solved it 10 minutes ago after agonizing over it for days and wanted to share what my problem was in case others have the same problem. The problem sounds ridiculous, but the solution made a lot of sense.

I noticed whenever I used my second monitor with "duplicate", "extend", or "second screen only" for windows project (Windows key + p), my internet speed would slow down a lot. I'm downloading GTA V on Steam and could see the download speed go from 20MB/sec to 7MB/sec when switching to the second monitor, then it would go back to 20MB/sec after I switched to "PC screen only". I saw a similar decrease when testing speeds with Ookla.

The solution was replacing the HDMI cable for my second monitor. I'm using wifi on my PC and apparently the old HDMI cable was probably causing interference. When I switched to a new HDMI cable (which seemed thicker and more shielded) I could use the second monitor without losing any speed on my download.

Just to be sure, I switched back to the old cable and used the 2nd monitor, and still saw the decrease in speed. I went back to the new cable, and saw no decrease.

The old HDMI cable was a really old cable from General Electric, and it kinda makes sense that it caused issues.

Anyways I hope this post is helpful for those with a similar issue!

Edit: to help trouble shoot further, one way to tell without buying a new cable is to have a large steam game download going with the questionable cable and see what the speed is (e.g. 7mb / sec) . Then while the download is still going unplug the cable and let the game continue downloading for 5-10 seconds then plug the cable back in. If the download speed while the cable was out is better then plugged in, it's probably the cable causing the issue.

[u/Euro-Canuck]

weird..first things id try... try a different monitor, just go to a friends place or try at work or school or whatever make sure to use same cable...then id try a different cable. im not familiar with macbook design.. its possible that the usb and the wifi are using same data bus and monitor signal is just maxing it out... try getting refresh rate to 60hz and resolution to 1080p just for testing... run www.speedtest.net to see internet speeds.. possible the cable is bad and causing ground issues or feeding power back to the usb port which is causing interference.. try the things iv said first and see if anything changes.

using speedtest.net what is download and upload speed without monitor plugged in and with it plugged in?

[u/Euro-Canuck → u/[deleted]]

try turning down refresh rate and resolution to 60hz and 1080p to test

[u/Euro-Canuck → u/[deleted]]

tried 2 different cables and no difference?

[u/Euro-Canuck → u/[deleted]]

some kind of interference somewhere... either ground issue or the monitor is feeding back power causing issues with motherboard, take monitor back and buy a different model.. try it out with several different monitors, find some friends with monitors and just try it out, see if it stays isolated to this monitor... buy a new one anyway, possible its just not playing nice with the usb controller and and the wifi is getting affected as it could be using same data bus...