Little's law
In any stable system, the average number of items inside (L) equals the average arrival rate (λ) times the average time each item spends inside (W): L = λW. John Little proved it in 1961, and it holds whatever the arrival pattern or the service order. For a server, requests in flight = requests per second × seconds each request takes.
Performance
In Quorum#
An App Server that holds a worker or a thread per request has a fixed number of slots, and each request holds its slot for its CPU time plus every database call it waits on. Slots in use = arrival rate × that time, so when each request waits 10 times as long on the Database, a PHP-FPM pool of 5 workers serves up to 10 times fewer requests. Fix the slow call, add workers or threads, or move to a runtime that waits without a slot (Node.js, Go, Kestrel, BEAM, Tokio).