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If it is a few specific queries and you have already gone through the general cluster tuning and segment partitioning/optimization exercises, then if you post your query here you should be able to get some help. I personally know a fair amount about SQL level tuning ... I know some segment partitioning/optimization, but others here know more ... I don't know of a specific "slow query perf tuning" guide available to the public ... but I suggest check out the Imply (Druid) YouTube channel, there are a lot of architecture tech-talk videos, some of which may cover aspects of tuning for query performance.

Flame graphs might be a good starting point. Imply (my employer) has also recently published a free course on Druid metrics, and I think one of the labs focuses on identifying a slow query.