Why important?
Price, stock, new products and deleted items must work in search time.
Glossary
Index Freshness
Index Freshness describes how quickly changes in the source data become visible in the search index and thus in results, filters and availability.
Measure the chain
Compare source change, synchronization, index processing and the time when the change is actually found.
Not just average time
Also check laggards, failed updates and vary by store or data source.
Operational KPI
Freshness should be followed together with event health, latency and availability in search operations.