Vitest runs every test file in a separate worker, which enables parallel execution and speeds up tests by default.

Each worker has its own isolated environment, meaning that if one test file mutates a global variable, other test files won't be affected by this mutation. This test file-level isolation is a great feature for preventing test interference.

While test isolation provides benefits, it comes with a performance cost.

When you have hundreds or thousands of test files, the overhead of spawning a worker for each file accumulates and can significantly degrade performance.

Follow the instructions to disable test isolation and observe the performance difference it makes in projects with a large number of test files.