When enabled, StockIQ calculates lead times based on order and receipt history.
StockIQ gets a feed of supply order and order line history (Supply Order, SupplyOrderLine). In order to calculate actual supplier lead times versus the expected values, we require information about shipment and receipt history, the feed for which is called "IncomingShipment", and contains dates such as shipment dates, delivery dates, etc.
Lead times on a shipment-by-shipment basis are calculated according to the Shipment/Receipt-based Lead Time Calculations article.
If there are enough receipts in a given time period for a given item-site-supplier combo (or one of the fallbacks), the minimums for which are specified in the Global Lead Time Settings, then the average lead time values for each of your admin, manufacturing, shipping, and putaway lead times are used as the calculated lead times for that Item-Site-Supplier combo.
Note that the final number StockIQ will use as the calculated lead time can be capped by the "Max Percent Change" relative to your baseline ERP lead time in the Global Lead Time Settings. If you find that StockIQ is not adjusting the lead time far enough in response to real-world lead time, then you can increase that percentage to give StockIQ more leeway.
Details of the final result calculations can be see in the "Lead Time Calculations" tab of the Lead Time Viewer.