This sounds like your desktop computer is simply hogging all of your internet bandwidth.
There are several reasons why this could be happening.
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When your internet s…l…o…w…s…
The desktop computer may be trying to upload or download something (probably a very large file or maybe several things). In doing so, it’s taking up all of the available internet speed that your ISP is giving you.
This could be temporary. For example, if the desktop is downloading updates, then the speed should return to normal after the download is complete.
It could be malware. Malicious software on that desktop could be trying to upload and download constantly to the exclusion of everything else.
It could be a router. Normally, your router does a better job of getting everything more or less equal time, but that’s actually an impossible task depending on exactly what’s happening. The concept of equal time doesn’t really apply. Some routers are simply better at it than others.
What do I do?
Run the diagnostic tools outlined in that article and see exactly what that desktop machine is trying to do. From there, you could probably make some decisions about turning off software or allowing downloads to run. But that’s where the information is and that’s where I’d have you start.