Speedup the RSpec test suite feedback cycle

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 is more performant but I'm not sure if this project is alive)
• using test watcher like Guard and 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.