I don't know if the slow downloads of Big Sur are related, but the underlying problem is that ocsp.apple.com[1] is fubar, and certificate revocation lookups are failing.

EDIT: This might indeed be Big Sur-release-day related. Most certificate revocation failures are "soft", but with ocsp.apple.com black-holed in /etc/hosts I can't resume downloading the update.

[1] https://twitter.com/lapcatsoftware/status/132699029641299148...