I have an ADSL router.
It works fine.
When I plug directly into it via ethernet, all is fine.
It doesn’t have enough ports though, only 4.
I bought a cheap Dlink unmanaged switch.
When I connect to the pc > switch > router it works, but internet access is noticeably slower. When I take the switch out its back to normal.
I’ve tested all cables with a cable tester.
Normal network shares are fast to access.
Found the issue, cable length reports as 200meters on Intel Network Diagnostic tool. All pins report as connected with cable tester. Used another cable just to be sure and it fixed the issue. I’m guessing the cable has an intermittant break or just a bad batch. Replacing with a new cat 6 shielded cable.