magento2: [Performance] CatalogSearch Indexer Performance Degradation with Elasticsearch

Preconditions (*)

1. Magento 2.3-develop#fcf9eed8a2456a88d36cfcca02f6a35ad81d6972
2. Install medium performance profile magento setup:performance:generate-fixtures setup/performance-toolkit/profiles/ce/medium.xml
3. Install Elasticsearch 5

Steps to reproduce (*)

1. Validate MySQL is enabled for search
2. magento indexer:reindex catalogsearch_fulltext Using single thread
3. Note index time. Locally I’ve observed ~4 minutes
4. Configure search to use Elasticsearch5+
5. magento indexer:reindex catalogsearch_fulltext Using single thread
6. Note index time. Locally I’ve observed ~22 minutes

Expected result (*)

1. Full re-indexing time should be somewhat consistent using different backends.

Actual result (*)

1. Using Elasticsearch increases re-indexing time by 5x.