WP Fast Search

Responds to key press, instantly presents a drop-down menu of post titles for the user to select from (with ten-post max per result set).

The user can scroll through the list of posts using the mouse or keyboard (pressing enter or clicking on a title will take the user to the selected post.)

Data for posts are loaded asychronously once the page is loaded, and so it will not affect the page loading speed. Only one request per page load is ever sent to the server, which queries for cached post information, and therefore the plugin is performant.