There are a number of books published now on Web Performance. Here is a summary of the books available on the topic, mainly from O’Reilly.
Delta encoding (also called delta compression) is a way to update web pages by sending the “diffs” or deltas between versions of a web page. The server (proxy or origin) sends only what has changed in the page since the last access. This technique greatly reduces the amount of data that is sent from server to browser. In some cases with pages that change slightly, the deltas are on the order of a few TCP/IP packets. As about 32% of page accesses are first-time visits, about 68% of page visits are eligible for delta compression.
First introduced by Internet Explorer 4, the defer attribute of the
script element is now part of the HTML 4 and XHTML specifications. The defer attribute gives a hint to the browser that the script does not create any content so the browser can optionally defer interpreting the script. This can improve performance by delaying execution of scripts until after the body content is parsed and rendered.