Apache Druid

If you configure your segments to have 2 or more replicas (which is recommended), you would need at least as many historicals as replicas.
A single historical setup wouldn't be resilient to historical failure and could lead to data unavailability.

<https://druid.apache.org/docs/latest/operations/basic-cluster-tuning.html#number-of-historicals> recommends having a smaller number of large servers provided that you have sufficient fault tolerance

the merge across segments in the historical is single threaded for a query (unless the aggregation spills to disk in which case it can be multi threaded). So having more than one historical gets more merge threads for a query

For faster startup you can try setting `druid.segmentCache.numBootstrapThreads` to the number of CPUs on the machine. Depending on how you are splitting the smaller nodes they might be using more threads as they might see more processors (Kubernetes for example normally reports the total node CPUs).