Use node_load via Ajax
Ajax could actually complicate your situation further, since you'd be asking your site to bootstrap once again asynchronously in order to serve the node. Not to say it isn't an option, you can write your own method to retrieve only the information you need via ajax.

I would personally look into caching methods. If your site gets anonymous traffic, try boost. Easily the best improvement for Drupal performance (in my opinion) is APC cache. Hopefully you have root access and can install the APC extension for PHP and then the APC module, the site starts flying after that, for both auth and anon traffic. I'm afraid you might already know this so I'll stop there.

If you still wanna do AJAX stuff, you could install views datasource to output JSON, and add a jquery call the page with $.get and push the content into the DOM. Using views gives you some native caching options as well.

Hope it helps.