Performance seems back to normal after:

  1. Back up configuration (not necessary, but a good precaution)
  2. Repair installation through Account->Settings->Repair Avast! One
  3. Reboot

YouTube and YouTube TV connect at appropriate resolutions for a 1 Gb/sec up/down network. Other websites operate normally (no stuck loading various parts).

I will see how this goes after an evening of work / play.