For IndexReader implementations that use TermInfosReader to read terms, this sets the indexDivisor to subsample the number of indexed terms loaded into memory. This has the same effect as {@link IndexWriter#setTermIndexInterval} except that setting must be done at indexing time while this setting can be set per reader. When set to N, then one in every N*termIndexInterval terms in the index is loaded into memory. By setting this to a value > 1 you can reduce memory usage, at the expense of higher latency when loading a TermInfo. The default value is 1.
NOTE: you must call this before the term index is loaded. If the index is already loaded, an IllegalStateException is thrown.IndexReader Class | Lucene.Net.Index Namespace