Can you take the ps4 to the router, connect direct with ethernet cable and not homeplugs. Turn it on as you do when it slows things down... Is it still slowing down the WiFi?
Sorry, i was only able to do this today.
It does slow down other devices but not as slow as using the homeplugs. it drops to around 7-15 after doing a few tests on speedtest.net. still all fine on this websites speed test.