How large is your project?

Without process isolation we got 6x speedup vs. jest! But it is difficult to ensure a clean slate between our test suites (~40 suites which all use a test DB)

We are also using some helpers to mock, e.g https://github.com/marchaos/jest-mock-extended - its such helper libraries im concerned about.

A young project and a small team (3 months, 3 developers).

We're working with a relational db, and we're DELETE FROM all tables that we use at the beginning of each test. That's fast enough so far. Process isolation doesn't help you with state in out-of-process dependencies anyway.