Yarn takes 12 seconds to install all of our node_modules, npm@5 (latest) takes over 12 minutes.
This made it completely and utterly untenable for our CI system, so we switched to yarn and shaved 12 minutes off our CI builds. It's been fantastic.
Old results from about a month ago:
https://twitter.com/bertjwregeer/status/887450964420055043
npm 5.3.0: added 1991 packages in 538.289s yarn v0.27.5: Done in 58.42s.