How we cut a test suite time from 40 minutes to 4 by upgrading Ruby/Rails, fixing slow tests, and enabling parallelization, dramatically improving developer experience and CI costs.

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A Rails upgrade consultancy shares how they reduced a client's test suite runtime from 40 minutes to 4 by upgrading from Rails 2.3/Ruby 2.5 to Rails 8.1/Ruby 3.4, migrating from Test::Unit to Minitest, replacing factories with fixtures where appropriate, profiling and fixing slow individual tests, and enabling Rails' built-in parallelization. Key challenges included fixing test ordering issues, resolving race conditions caused by shared temporary files, and ensuring tests were idempotent. The post also covers alternative parallelization gems (flatware, turbo_tests, parallel_tests, knapsack_pro) for RSpec/Cucumber users. The result was a 10x speedup, reduced CI infrastructure costs, and a dramatically improved developer feedback loop.

From 40 Minutes to 4 With Tests Parallelization - FastRuby.io