19% of Tests Eat 87% of Runtime: a Pareto Principle Fix for Slow Test Suites
rchardkovacs · x · 2026-10-21
Developer rchardkovacs profiled his test framework and found a textbook Pareto distribution: just 19.1% of tests account for 87.2% of total runtime.
A screenshot shows the split after separating tests into two tiers: bun run test --fast runs 4,221 tests in 1.9s, while bun run test --spawn takes 12.9s for 998 tests; the full 5,219-test suite passes in 19s. Knowing which few tests dominate runtime tells him exactly where optimization effort pays off most.
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