Improving Drupal's page loading performance

Introduction
Google dominates the search engine market for a large part thanks to its spartan, no-bells-nor-whistles interfaces. But also thanks to its incredible speed (which is partially thanks to that spartan interface, of course).

Since you're reading this article, you're probably a Drupal developer. It's pretty likely that you've had some visitors of your Drupal-powered web site complain about slow page load times. It doesn't matter whether your server(s) are shared, VPSes or even dedicated servers. Visitors that live abroad – i.e. far from where your servers are located – will face the same performance issues, but at even worse scales.
This article is about tackling these issues.

Front-end performance
Faster servers with more memory stop improving your web site's performance at some point. Yet, even before your web site gets big, there are other places to look at to improve performance, where greater effects can be achieved, even at lower costs – significantly lower costs actually. Typically, less than 20% of the total response time is used to retrieve the HTML document. That means the other 80+% is used to process what's in the HTML file: CSS, JS, images, videos. And in many cases, that number is even higher.

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Courtesy : http://wimleers.com