Speed as differentiating factor

Optimization is based on improvements concerning network connectivity, the web server optimization and improvement of the different elements that have their own websites, from HTML to JavaScript, CSS or going by the number of requests to DNS servers.

Many stories about how large Internet companies have improved their businesses by increasing their websites performance are not so many years ago:

Amazon: 0.1 second delay means a loss of 1% of revenues.
Google: 0.4 second delay causes a drop of 0.59% of searches per user, 0.5 seconds to load involves 25% fewer searches.
Google Maps: reduced by 30% the size of their files and the number of requests increased 30%.
Bing: 1 second delay implies a fall of 2.8% of revenue, 2 second delay implies a drop of 4.3% in revenue per user
Yahoo!: 0.4 seconds of delay causes a drop between 5% and 9% of the traffic.
Mozilla: make their download page 2.2 seconds faster and there was a growth of 15.4% downloads.

In general:

47% of users expect a page load in under 2 seconds.
14% of online stores users change if the page takes too long to load.
40% of users leave a page that takes more than 3 seconds to load
64% of buyers who are not satisfied are changing their website for their next purchase

In summary: The more performance, the better. However, creating fast websites require optimizations in these aspects:

Connectivity
Contents
Cookies
CSS
Images
JavaScript
Mobile
Server

Large sites like Yahoo! or Google have launched various tools to check the speed and performance of web sites, whether from their web platforms and external applications that work in some browsers.

Google mod_pagespeed: is a module for Apache 2 loaded with the default settings of many options (compression, CSS and JS group…).
Google PageSpeed: a tool to test the speed of a website from your browser
Yahoo! YSlow: is a tool that can be installed in many browsers as an addon