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Average service time alone is not enough for capacity planning, because it is the variability that creates queues. Separating fixed setup from the variable component shows where time can be removed, and the 90th percentile is what determines the worst-case customer experience.
Average service time
Average = Total service minutes / Customers served
90th percentile
P90 = Average + 1.2816 x Standard deviation
Because queues form from the interaction of random arrivals and variable service. Halving variability reduces waiting more than shaving the mean by the same proportion.
Remove the fixed component first — pre-authorisation, pre-packing, self-service check-in — since it applies to every customer regardless of complexity.