At work we've tried switching to vitest, and it's 1.5-2x slower than Jest (I think it's related to our very large and circular module graph), so performance is very much a your-mileage-may-vary thing.

You'd be surprised https://github.com/artemave/node-test-runners-performance-co...

Say this again when you have worked with Jest, one of the worst and slowest pieces of software I've ever worked with.

You'd be surprised how slow the JS ecosystem can be.

> Surely all the time is taken by the tests, not calling the test functions?

Calling tests has overhead. Also knowing how to schedule and parallelize tests with dependencies is not as "simple".