synapse takes ages to restart

[matrixbot]

This issue has been migrated from #7968.

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Why does it take 70 seconds to initialise the database? It might be some sort of database connection contention but it would be nice if there were some log messages or something.

2020-07-28 09:56:32,141 - synapse.storage.data_stores - 50 - INFO - None - Preparing database 'master'...
2020-07-28 09:56:56,412 - synapse.storage.prepare_database - 325 - INFO - None - Upgrading schema to v58
2020-07-28 09:56:56,421 - synapse.storage.data_stores - 60 - INFO - None - Starting 'main' data store
2020-07-28 09:57:42,940 - synapse.config.appservice - 87 - INFO - None - Loaded application service: ApplicationService: ...

what's going on there?