Vector index shows as 'building' or 'initial sync' for extended periods (hours/days); queries using $vectorSearch return no results or perform slowly (e.g., 30s for small top-k); possible Atlas alert for 'Index Replication Lag'.

Not a confirmed bug. Long index build/rebuild times are expected for large vector collections (millions of embeddings), as Atlas must scan and index the entire collection. Insufficient memory causes slow performance post-build; replication lag in search nodes (mongot behind oplog) can stale indexes, triggering rebuilds if severe ([Atlas Search Alerts](https://www.mongodb.com/docs/atlas/reference/alert-resolutions/atlas-search-alerts/)).