That's a bit of an open question right now, unfortunately. Using S3 to store Lucene indexes is a roll-your-own thing since last I checked, and the implementation I wrote currently deals with smaller indexes where files can be pulled to fully disk as needed. S3 does support range requests, which I'd think would mimic random access well enough.

Assuming whatever ElasticSearch implementation you're using is backed by SSDs there'd likely be more latency with S3, but I'd expect it to scale pretty well. Internally, a Lucene index is an array of immutable self-contained segment files that store all indices for particular documents. Searching in multiple indices is pretty much just searching through all their segments- which can be as parallel as you want it to be.

To be honest, I'm actually surprised the Elasticsearch company doesn't offer this as an option. Maybe because they sell hardware at markup?