I'd prefer that all testing be done with live data in a real database ... maybe not at the very beginning but as soon as practicable. I came into a project recently with so many issues around differences between real data and assumed data, and unrealistic assumptions about real data that it's been hugely painful, with lots of back and forth between Dev and QA and Ops. With Docker now there's no excuse for not setting up soon in a project's lifecycle a realistic environment (infrastructure topology-wise) with real data on development and QA laptops.

With this you still have fast and slow perhaps, but everybody can run all but the most extreme scaling tests.