Flat_object performance vs object

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser): 2.11 / AWS

Describe the issue: When performing large bulk inserts (6 million entries) for relatively simple objects (address data with 5 string fields) flat_object seems to perform ~50% worse than object. With flat_object on similar hardware we get 400k inserts / min while with object we get 900k inserts / min. Is this to be expected?

While documentation doesn’t explicitly state that flat_object would perform better when indexing, it’s easy to make assumption that it would (indexing one item vs indexing multiple items).

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