We've discussed previously the negative impact of large bulk operations, especially wrt WAL writes. Patch here allows maintenance operations to have their WAL generation slowed down as a replication lag prevention feature.
I believe there was originally intended to be some work on I/O rate limiting, but that hasn't happened and is in some ways orthogonal to this patch and we will likely eventually want both.
Single new parameter works very similarly to vacuum_cost_delay
wal_rate_limit_delay = Xms
Seems like a really bad name if we are only slowing down some commands - that seems to indicate we're slowing down all of them. I think it should be something that indicates that it only affects the maintenance commands.