Yes, ES uses multiple threads nicely. But as you move to 32 or 64 cores - in my experience - a single threaded client won't keep ES/Lucene busy enough.

With SOLR, it's similar:

> Sometimes you need to index a bunch of documents really, really fast. [...] The solution is two-fold: batching and multi-threading

From: http://lucidworks.com/blog/high-throughput-indexing-in-solr/