How does document size affect ElasticSearch performance?

Planning to increase document size in elastic, from 4KB to ~100KB each. What is the performance impact with document size increase?

The new fields will have to be additonality used to calculate/modify final scores of documents that are retrieved.

Planning to keep doc_values true for all the newly added fields.

How are you querying? Are you primarily running aggregations or doing searches? How many documents do you typically return? What is driving the increase in document size?

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Search + aggregation both. The newly added fields will only be used for script_score though.
We return around 30 to150 documents based on use case.

We are adding 4000 extra keys which would ultimately calculate/modify the final _score of ES docs.

Our cluster is 20 data nodes 200 GB disk size each.

Reading significantly larger documents from disk could affect performance. It is impossible to determine how much impact it will have though, so I recommend you run a test or benchmark using as realistic data and load as you can to find out.