Possible causes of high_replay lag, given replication settings?

Hi,

I just had a situation where physical replication fell far behind
(hours). The write and flush lag times were 0, but replay_lag was
high. The replica has hot_standby_feedback on, and both
max_standby_streaming_delay and max_standby_archive_delay are set to
30s.

What could cause a situation like this? If the network were a problem,
I'd expect the other _lag times to be high. So it appears that the
replica was getting the WAL but was unable to apply it. Are there
situations where the replica cannot apply WAL other than the kinds of
conflicts that would be addressed by the _delay settings?

I checked pg_stat_database_conflicts, but there was nothing in it -- all zeros.

- Jon