VDSL or Fibre?
Neither, it's DOCSIS 3.
It has less functionality in this regard than the firmware that came with the router (a Linksys WRT610N). At least that automatically configured 6to4, even if it still didn't have any GUI options.
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.