When doing the trains, I keep screaming cause the sync wikiversion of testwiki is very long. The main reason is that we generate the l10n update code at that point.
When rolling the train, we do a scap prep XXXX which sync the code everywhere. We should be able to generate the l10n cache solely for the new version and sync it along the code as a preliminary step.
Then when we want to deploy to the test wikis, it will be just syncing wikiversions.json and should be lightning fast. That would help doing all the preparation outside of the train window and work toward further automatizing the train.