If you are changing the index definition, then you need to manually reindex it otherwise incremental indexing is performed based on the type of index:
async -> Every 5 Seconds
fulltext-aync -> Every 5 seconds
nrt -> Real-time index update with no delays
Also, Changing the "Async Indexer Configs" at "OSGi config -> org.apache.jackrabbit.oak.plugins.index.AsyncIndexerService" will have a performance impact if you change it to less than 5 and you might not get expected results if you change it to more than 5 as almost every AEM console uses aync,fulltext-aync index types.
This config should only be changed if recommended by Adobe Engineer after analyzing the complete use case.
But this will not provide informaiton specifically about your custom index.
Debug level logs for the appender: org.apache.jackrabbit.oak.plugins.index.AsyncIndexUpdate and org.apache.jackrabbit.oak.plugins.index.IndexUpdate might be helpful to know whats happening with reindexing. This would really fill up logs though.