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Test parallelisation is bounded below by the single longest test, because no split can finish faster than its slowest indivisible unit. Once total work divided by workers falls below that duration, extra workers only add setup cost. The maximum useful worker count is therefore total time divided by the longest test.
Parallel test time
wall clock = max(total / workers, longest single test) + setup per worker; max useful workers = total / longest test
Because tests are the unit of scheduling. A 90-second test occupies one worker for 90 seconds no matter how many other workers are idle.
By recorded duration, packing workers to equal time rather than equal count. Most runners support timing-based splitting from a previous run's report.