Why lead time is not processing time
Where the waiting happens, and why it dominates the total.
Lead time differs from processing time because the hours a part spends being cut, welded or assembled are only a small piece of the total time between an order being placed and a finished part arriving, and almost everything in between is the part sitting still, waiting for something else to become free.
Goldratt's The Goal was recommended as required reading for exactly this, an odd claim for a novel about a struggling parts plant, and it took less than a chapter to see why. A plant manager slowly discovering that his output was throttled by a single overloaded machine, illustrated by a scout hike whose pace was set by its slowest walker, lands harder as a scene than it would as a formula on a slide.
Five minutes with the doctor, an hour at the surgery
A routine visit to a doctor's surgery can stretch to most of an hour once travel, checking in and the waiting room are counted, while the consultation itself takes a few minutes. Nobody would describe the appointment as taking five minutes. A part moving through a factory is in the same position, and describing its lead time using only the processing minutes understates the wait in the same way.
Processing time is short and predictable, bounded by how fast an operation physically runs. Queue time has no such ceiling, because it depends on how busy everything ahead of the part happens to be when it arrives. Asked how long a part takes, the instinct is to add up the times on the routing sheet, since those are the only numbers anyone measured on purpose, and the waiting between operations, which nobody owns the job of measuring, never appears in the total. Little's law ties the two together, relating how long work spends in a system to how much work is sitting in it at once: a queue holding more unfinished work takes longer to clear, whatever speed each item is processed at once its turn comes.
One shift of cutting in three weeks of waiting
A component machined in a single eight-hour shift but delivered three weeks after the order has spent eight of roughly five hundred elapsed hours being worked on, under two per cent. Halving the machining time saves four hours against three weeks, which barely registers. Shrinking the queues themselves, with smaller batches, a machine freed up sooner, or work released to the floor at a steadier rate, can cut days, and a faster machine disappoints for the same reason, since parts still queue in front of it almost as long as before.
Some lead times are dominated by the process: a casting that takes days to cool, a coating with a fixed cure time, a heat treatment governed by chemistry. Telling the cases apart means measuring how a part's time splits between being worked on and waiting.