I decided to compare if there will be significant differences between dockerized Postgres and non-dockerized. Here is my test environment: rails application with rich test suite, about 628 examples, macOS Sierra 10.12.6, Docker 17.06.0-ce-mac19.

I use Postgres docker version alpine 9.6 which is 37,7Mb and raw Postgres 9.6 with GUI for Mac which is 379Mb.
Here is my docker-compose config

db:
  image: postgres:alpine
  ports:
    - 5432:5432
adminer:
  image: adminer
  ports:
    - 8080:8080
  links:
    - db

With docker
Finished in 46.86 seconds (files took 17.77 seconds to load)
628 examples, 0 failures

Without docker, raw Postgre 9.6
Finished in 31.35 seconds (files took 8.38 seconds to load)
628 examples, 0 failures

And again in Docker
Finished in 41.64 seconds (files took 8.24 seconds to load)
628 examples, 0 failures

And again without Docker
Finished in 31.53 seconds (files took 8.01 seconds to load)
628 examples, 0 failure

And again with Docker
Finished in 41.77 seconds (files took 8.51 seconds to load)
628 examples, 0 failures

So its 41,5 seconds for Docker version and 31,53 seconds without Docker average.
This is 24% difference in particular rails rspec case, which is quite significant as for compartion and is not so significant for testing loop.

At least this is a price you pay for container portability.