Recently, we were debugging some performance issues with a client's Drupal Commerce website. After doing the standard optimizations, we hooked up New Relic so we could see exactly what else could be trimmed.
We're big fans of Elasticsearch. It is significantly easier to deploy, manage, and scale than SOLR in our experience. Since we've already been using Elasticsearch for indexing and storing system logs, it makes since for use to use Elasticsearch as the search backend for our clients' websites.