Delay in seconds for the indexing thread pool to sleep after a file has been indexed in lazy mode. Alkaline goes into a so-called lazy mode when responding to search requests.
When you specify this option with a -mi parameter bigger than 1 (which is the default), you might see no effect of this option because it defines the sleep interval of each indexing thread rather than across all indexing threads. Use -mi=1 to obtain the correct effect of sleep interval.