Work out test suite runtime instantly with clear inputs, formula shown and shareable results.
Suite runtime is dominated by the slow layers, not the numerous ones. Four thousand unit tests at 12 ms run in under a minute, while 380 integration tests at 850 ms each take over five. That asymmetry is the whole argument for the test pyramid: push assertions down to the fastest layer that can still catch the defect.
Suite runtime
total = unit count x unit duration + integration count x integration duration
They pay real I/O: process startup, database connections, network calls and fixture setup. Each of those costs orders of magnitude more than an in-process assertion.
Share expensive fixtures across tests, run against in-memory or containerised dependencies, and reserve full end-to-end tests for a small set of critical journeys.