Hi – thanks for the good question – firstly – Elastic Search is an awesome product, but all of the plugins I’ve seen for Elastic Search focus on replacing the ‘search’ feature of WordPress. Specifically, they replace the WP_Query search with a search against Elastic Search.

The benefit of this is that you get far more relevant results as Elastic Search uses a proper full-text search index which provides relevance, stemming, synonymns and other useful features for text-based search.

In order to use Elastic Search you’re going to need some technical know how – firstly to install it and secondly to get it working. There are some plugins available, but nothing I’ve seen which is just plug and play.

The WPI Performance Plugin on the other hand works with the existing WordPress tables and optimises them. At the time of writing it doesn’t modify the free-text search, although this is in the pipeline and will still use MySQL full-text indexes (both available on MyISAM and InnoDB tables now).

As it stands right now, the WPI Performance plugin optimises queries against postmeta tables, taxonomies and posts (through both query modification and binary-tree (not full-text) index addition). That means it gives speed bosts all over the place. As it currently stands, this excludes free-text search which is still using the default LIKE operator – and this is the primary benefit of server products like Elastic Search, SOLR and Sphinx. But – as I said – an update for this plugin is in the pipeline which adds full-text indexes to wp_posts and modifies the WP_Query object to use that full-text index providing both faster and more relevant results.