Shard Lucene segments single threaded?

I am just curious, I know a search on 5 shards take 5 Java threads, but if each shard have 10 segments:

  1. Each segments are searched synchronously ? (mean the more segments the longer is the search)
  2. Each segments are search in parallel ?

Thanks you,

(I search arguments to recommend elasticsearch, from Q&A from "The Great Search Engine Debate - Elasticsearch, Solr or Vespa?" Meetup | Vespa Blog ^^)

Answer is 1. That's why having only one segment for a stable index is better.