Currently 3 search engines exists :
eZ Publish Platform 5.4.5 and higher, Solr is the recommended search engine for production use of the API. Solr provides the most advanced search capabilities, and system performance. Currently Elastic Search is experimental, not professionally supported, and won't be made stabile anytime soon.
Note: Solr Bundle does not support advance search features like out of the box facets (making custom facets is possible), spellchecking and more like this, for that continue to use ezfind as those features will most likely not be backported to 5.4 (Bundle and kernel). Recommendation above is regarding speeding up the API as a lot of API interactions is using search api, which gets more stable performance when running on Solr.