While w3d's statement that both F5 and enter will use the cache is technically mostly true, it is misleading. The devil is in the details.
When pressing enter, the browser may load any resources from cache without rechecking them depending on the expiration time (http
Expires header) and other http headers. This means that most resources can be loaded as quickly as the disk or RAM can respond, potentially less than 1 ms if the resource is in RAM.
Pressing F5, on the other hand, will always send a request to the server with an
If-Modified-Since request header with the timestamp of the currently cached version of the resource. The server will then either respond with a
200 OK status code followed by the data, or a
304 Not Modified status code. For a static resource, this will most likely be a
304 status and the browser will load the resource from cache. In other words, the browser is not allowed to load the resource from cache until it's gotten a response from the server. So when pressing F5, even though the resource might not be fully retransmitted, the load time of the resource is still subject to network latency and the server's response time. This could take 50-100 ms or more per resource.
This behavior can be observed with the network function of the browsers' built-in development tools. You can open the development tools yourself by pressing control+shift+I, selecting network and observing what happens when you reload the page in the different ways. In Chrome, you will see that when pressing enter, resources loaded directly from cache will have status
200 OK and size
(from cache). In Firefox, resources loaded directly from cache are not even shown in the network view. When pressing F5 on the other hand, requests are sent to the server for all resources, which will mostly respond with a