Tuning Elasticsearch is an advanced topic, that attending in an Elasticsearch Engineer Training by elastic is pretty simple to understand. The usual answer is always:
It depends. I summarise my notes and findings on this. Consider there is always a lot more to it. The training covers it all. It is undoubtedly worth to attend. Education is important ;-). Roughly it is about Elasticsearch Internals (Apache Lucene under the hood), Capacity Planning and smart choices/anticipation.