Use of Lucene Indexing can significantly improve performance and enable additional functionality on terminology operations, so it is recommended for use in environments where validation or other terminology operations will be heavily used.
Smile CDR supports typical validation use cases out-of-the-box without Lucene Indexing (e.g. "Is this LOINC code valid?"). Additional indexing is only required to support more elaborate validation use cases (e.g. "Is this code a valid part of the substance hierarchy in SNOMED CT?").
Smile CDR supports all terminology operations out-of-the-box except for the $closure operation. However, Lucene Indexing is used to support ValueSet expansions that involve properties and hierarchies. If a solution needs to make use of the filter element of the ValueSet resource, Lucene Indexing is required.