VDSL or Fibre?
Neither, it's DOCSIS 3.
That would explain why my recaptcha protected forum suddenly started getting 30+ new accounts a day.
TCP isn't the problem. TCP relies on packets being dropped to know when to back off (it's the only way it can know the end-to-end bandwidth available). The network has masses of buffers now, nothing gets dropped in a timely fashion, so TCP keeps on sending.