As pointed out by Wired,the feature works on slower connections, so that users who most need a greater degree of precision in their searches–that is, those with, say dial-up connections, whose every click can cost precious time–will get it. Hence no messing around for hours, trying to find the information one’s after. Sweet for users, eh? Sweetest, however, for Google.
Here’s why. Instant Preview will potentially allow Google to own more of our browsing experience. Instead of leaving Google in order to browse the web, you will be browsing the results, rather than the pages suggested in the search itself. The feature may also force websites to redesign their pages in order to conform to Google’s design criteria: on the firm’s Webmaster Central blog, this is one of its suggestions.
Try to avoid interstitial pages, ad pop-ups, or other elements that interfere with your content. In some cases, these distracting elements may be picked up in the preview of your page, making the screenshots less attractive.