The circleci/postgres image is using regular persistent storage on disk. Using a tmpfs for example would make tests run MUCH faster and would even cost you less resources
I created a Dockerfile for my own project which reduced the time our tests take to run from 130s to 27s with just one simple line. You should consider adding it to your recommended image.
ENV PGDATA /dev/shm/pgdata/data
So the PGDATA env should be exported in your recommended version maybe. Any thoughts on that?