I haven't read all the posts, I have a 12 year old PC, 3BB 631 bht 1GB supply.
Speed test, 750/305Mbps.....
Only prob I had in the past was not getting anywhere near 1GB, the LAN card in the PC was not 1GB, replaced it to 1Gb which cured the prob.....
A top-notch VPN pulled the speed well down, did away with it, the 3BB supply shows my IP different all the time, not sure why, but that's OK....????
So check LAN cards speeds...