How to run parallel jobs for RSpec tests on GitLab CI Pipeline and speed up Ruby

The common problem, when you want to run tests in parallel to complete your 1-hour test suite in a few minutes instead of waiting hours, is to find a way how to split tests on parallel jobs. Some of your Ruby or JavaScript tests can take milliseconds and some even a few minutes per test file (for instance when using Capybara in RSpec features testing). Problem with slow tests also occurs in E2E (end to end testing) when using Cypress test runner as browser testing can take quite a long time to execute.If you add more parallel GitLab Runners you also may notice that some runners can start work later or not all jobs can be started at the same time (for instance when you run GitLab Runners on your own infrastructure and other CI builds occupies some of the runners).