Why is Tantivy faster than Lucene?

EDIT (2025-11-12): Thanks to learnings from the Tantivy project, as of version 10.3, Apache Lucene is now on par with Tantivy on most queries and even faster on some queries. This is likely a temporary win though, until the Tantivy project replicates performance improvements from the Lucene project, or even finds new opportunities for optimizations!