Speedup the RSpec test suite feedback cycle
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The tests running speed is pretty low. `spring` speeds up things but the situation seems far from perfect (comparing to nowadays feedback speed for, for instance, FE test suites like Jest).
I was thinking of:
- using different preloader (from my experience, [`Zeus`](https://github.com/burke/zeus) is more performant but I'm not sure if this project is alive)
- using test watcher like [`Guard`](https://github.com/guard/guard) and [`guard-rspec`](https://github.com/guard/guard-rspec)
- turning off the SASS compilation for the view specs
- etc.
This issue is for the discussion and sharing thoughts on the performance of the test suite and its feedback speed.
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