Parallel Test Execution
Parallel test execution runs multiple tests simultaneously across different processes, machines, or browser instances instead of one after another, cutting total suite runtime dramatically. It's what makes large automated test suites practical to run on every pull request instead of only overnight.
A suite of 1,000 tests running serially at 2 seconds each takes over half an hour; split across 20 parallel workers, the same suite finishes in roughly 100 seconds — the core reason CI pipelines invest in parallelization infrastructure as test suites grow.
Parallelization surfaces test independence problems that a serial run can hide: tests sharing state, a database, or a fixed port will interfere with each other when run simultaneously in ways they never would running one at a time — which is why properly isolated test data and environments matter more, not less, as parallel execution scales up.
