As you probably know, Google recently rolled out its Instant Previews feature. The new feature involves quite a lot of client-side JavaScript, yet doesn't slow the search engine down for most of us. The Google Code Blog has a post on how the development team managed to add the new feature without noticeably impacting performance.
Here's what Google uses to keep the speed up:
- JavaScript compilation with Closure Compiler
- JSONP instead of XmlHttpRequest.
- Data URIs instead of plain images
More details can be found in the original post.