Modern websites need to load quickly – they do this by caching. This means web documents, such as HTML pages and images, are temporary stored (cached), to reduce bandwidth usage, server load, and perceived lag. Web caching retrieves the page a visitor accesses from our website and stores a copy, either via their browser or via our website firewall.
The next time the user will experience a very fast download because of caching.
See also:
Why can't I see the update I just made on the live site?