The move was somewhat chaotic.
My impression is that everyone got the move message immediately. That overwhelmed my $39/mo (!!) masto.host instance, which started rate-limiting.
Retries were scheduled without any jitter, so all the ones that failed came back at the same time, overwhelming the instance again. Rinse and repeat for a few hours.
Not sure what would have happened if the retries failed too many times, as everyone immediately unfollowed the old account ?♂️