Joe Obbish takes on parallelism:
You may have noticed large variations in elapsed time for parallel queries while performing query tuning or observing a production workload. This post reviews some of the possible explanations for those variations. I’m testing on SQL Server 2022 RC0 but this behavior can be observed on all currently supported versions of SQL Server.
Joe has an interesting example (and my guess of how bad the performance would be was not too far off, though I did underestimate the difference), as well as several possible causes and mitigation strategies for slow parallel queries.