2.4GHZ wifi drops and poor performance when a wired connection is active

[#1]

My partner's laptop uses a 2.4 ghz wifi connection. Her best download speed is around 12mb.

I use a 5ghz wifi connection on my newer laptop from the same location and get around 20mb.

My son, downstairs, connects a desktop via ethernet, My partner's internet connection becomes intermittent when he is connected and her speed drops to no more than 2mb while my connection is unaffected. When he disconnects, her service returns to normal.

I have no explanation for this. Has anyone else experienced similar and can offer any advice on how to fix it?

Thanks in anticipation!

Chris

[#2]

Is the desktop PC directly wired with ethernet to the hub?

How long is the ethernet cable?

[#3]

What router Chris?