Short answer: because we are obsessed with it every minute of every day!
Long answer: by analyzing all the layers/components with milliseconds in mind--the network, the hardware stack and obviously the software stack. No compromise! Everything is designed to be highly efficient, and our goal is to make it impossible to do better! For example, we embed our query code directly in nginx using a C++ module. In 99% of use-cases, the index is directly memory-mapped and the query is executed on the serialized data-structure on the fly; the result is then directly returned by nginx.