Redshift has some pretty significant and fairly unpredictable per-query overhead. For example, all queries are compiled, then code is distributed to every node

Redshift has some pretty significant and fairly unpredictable per-query overhead. For example, all queries are compiled, then code is distributed to every node in the cluster, then executed and gathered before returning. And most analytics databases use a scheduled execution model, which can delay execution from a couple seconds to several minutes, depending on database loads.