Caching

Caching is the practice of saving a response the server already generated so it can be handed out again without redoing the work. The first visitor triggers the effort of building a page; everyone after gets the saved copy almost instantly.

It is one of the most effective ways to make a site fast and to reduce strain during traffic spikes. The tradeoff is freshness: a cached page can go stale, so part of the job is deciding how long to keep a copy before generating a new one. CDNs rely heavily on caching to do their work.