Web Performance is a vigilant task and even well monitored websites frequently get out of performance monitoring radar resulting in performance regression pushed to end users. Real User Monitoring is an essential tool but it's not enough as users will be ultimately the ones raising the red flag for eventual performance regression introduced after every release cycle.
YSlow has been helping the industry websites performance by analyzing and suggesting the appropriate fix through browsers extensions manually run from productions end points. Now YSlow powered by NodeJS and PhantomJS is capable of been seamlessly plugged in the Continuous Integration pipeline raising the red flag for performance regression in the building process, assuring the website minimum performance standards and avoiding unnecessary end user bad experience. Besides YSlow as a way to prevent regression, at Twitter we strive to keep and improve performance of every new release through automated branch performance comparison tool powered by WebPageTest and WebPageTest API Wrapper which gives accurate metrics to avoid even milliseconds of performance regression in production.