My last project had a huge bottleneck at system acceptance test (SAT) stage - the SAT team was not able to sign-off stories at the same pace as the dev teams. Though I don't readily have the data, I am certain that the in-process time at the SAT stage was not high. Between the dev team completing the story and the SAT team picking it up were 2 steps - deployment into SAT Servers and showcasing these stories to SMEs and SAT team. Though we were deploying into SAT on a weekly basis, the showcases were done only once in 2 weeks.
There are 2 questions that bother me:
1. Is SAT the bottleneck? Is bottleneck identified purely by the inventory before that stage?
2. How can we ensure we manage the SAT process better?
1. SAT is not the bottleneck here. If we define bottleneck as the number of stories that can be "processed" at a particular stage, given their ability to work full-time, SAT was not the bottleneck - as their ability to sign-off stories was high (demonstrated during the later iterations of the release). Even if we consider inventory as a measure, given that showcase is a mandatory step before SAT and showcase is done only once in 2 weeks, showcase could be the bottleneck. SAT is like the final assembly. What consumes time is not the assembly itself, but the wait for all parts to come through before they start.
2. I reckon one thing that can be done is: reduce the batch size. Do more showcases. Once a week maybe.