I noticed that the "Restoring database schemas in the new cluster" part of pg_upgrade can take a while if you have many databases, so I experimented with a couple different settings to see if there are any easy ways to speed it up. The FILE_COPY strategy for CREATE DATABASE helped quite significantly on my laptop. For ~3k empty databases, this step went from ~100 seconds to ~30 seconds with the attached patch. I see commit ad43a41 made a similar change for initdb, so there might even be an argument for back-patching this to v15 (where STRATEGY was introduced). One thing I still need to verify is that this doesn't harm anything when there are lots of objects in the databases, i.e., more WAL generated during many concurrent CREATE-DATABASE-induced checkpoints.
