It's because of Passenger's built-in short-term HTTP cache, which is included in the open-source version. That said, the performance gain from the charts is from caching; it's not going to make your Ruby code run 4x faster. Be sure you read https://blog.phusion.nl/2015/02/09/turbocaching-security-cha... to make sure you don't accidentally end up serving things from a cache that you intended to not be cached/shared.