updateResultsToJsondoes an update per row, and doesn't do any batching for updates. This causes high latency due to the large number of update queries talking to the database when dealing with multi-million rows tables.
This makes '2.1.0 Alpha, step 18' potentially much slower then the database is separate from the php process. During a recent migration, this took ~3 hours compared to an hour for that step on an identically spec'ed single VM. While there is the 'zombie load' patching eat more performance, it shouldn't eat that much of it.