Something from PR #14675 is causing extension activation to hang, but only when users have explicitly opted in to the "discovery" experiment (or "All"). There are some reports that uninstalling pylance fixes the problem but that needs to be verified and may be unrelated. See #14977.

We've reverted #14675 in the December release branch but it is still in master. The problem needs to be resolved there soon and certainly before the January release.

Related: 73062ee