Comments on Gmail by some critics

'Gmail is slick, smooth and very, very fast. It's speed in displaying messages and switching between different views rivals that of desktop mail clients, and many of the features on offer (the innovative threading and the outstanding search capabilities) are leaps and bounds ahead of regular applications. This is a web app that for the most part works better than its desktop equivalents.

From the technical side of things, the performance boost is achieved using a particularly clever piece of JavaScript trickery. The bulk of the Gmail application is loaded in to memory in a hidden frame the first time you visit the site. From that point on, emails, thread listings and other views are loaded from the server as ultra light-weight JavaScript data structures. Bandwidth usage is minimal, and response times over broadband are virtually negligible from the user's perspective. Even the email address auto-completion (a particularly slick piece of the Gmail puzzle) calls back to the server on every key stroke!'

-http://www.sitepoint.com/blog-post-view.php?id=180963