Hi z1ts
I think your overthinking
The best way to test if your right, is to unplug the router from your network, turn off wifi at router level and then with 1 pc connected hard wired to router, repeat the speed test 3 or 4 times, then restart the router and repeat speedtest 3 or 4 times. Then use the average of the tests
If same ping and slow speed, do you have a different router to test with, to exclude your current router
Personally, my gut feeling is it's a daisy issue, but you need to exclude your router and network ideally before you contact your broadband provider.
Also, are you using a plug in filter or a nte faceplate with inbuilt filter
Many thanks
John