Peter actually makes this exact point in the project's README. See this section here:

https://github.com/peterldowns/pgtestdb#how-do-i-make-it-go-...

I'd just say that a nice thing about it on disk (even if you disable fsync) is that in case of a failing test, you can examine the post-run state which is occasionally extremely valuable.

Yup! If you keep your laptop on, failing dbs are investigable on tmpfs too — works fine for debugging a few tests in a loop, just can’t run the tests, sleep the computer, have lunch, and come back.