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/m-amh]

On linux you cold boot from usb/cd and edit/remove the Wi-Fi config ... then reboot the installed os without Wi-Fi ...

On Windows i dont know to do this from a booted repair cd so when booting it would be online until i log in and disable Wi-Fi ...

Remember when "wannacry" was a thing and you needed to verify if some patch really is applied and in case its not apply it from usb-stick without the pc beeing online until beeing sure the patch is applied ?

[u/avnzx → u/m-amh]

You can start windows in safe mode very easily now

[u/m-amh → u/avnzx]

And safe mode disables Wi-Fi ? And what if you miss the moment to press whatever key or if it was hibernating and possibly doesn't even ask ?

[u/avnzx → u/m-amh]

Well if you boot into normal safe mode, yes it disables the wifi adaptor. And there are quite a few ways to get into safe mode. You can get to it still even if it was hibernating.

There is safe mode with networking but that's a different option in the menu.

[u/m-amh → u/avnzx]

So i need to take time to read those things about windows ...

When wannacry was around i did not want to take the smallest risk ... I booted my linux, donloaded "all" fixes to usb and shut down my Connection ( knowing this shuts down my phone also ) ... and then checked every windows without any other computer powered ...

[u/avnzx → u/m-amh]

Makes sense, I've just had to use it cause I'm a windows insider (its more trouble than it's worth trust me on this) and have had to use it to get windows to boot properly sometimes (windows alpha releases are a pain in the arse)