Effectively using subodomains on your website will decrease your page load time. If your site is image-heavy your pages probably load slowly. Web browsers load all elements of a page from one TCP connection stream and each TCP stream only gets so much bandwidth. For example, if a page has seventy images to load (including your logo, nav items, etc), HTML content, java scripts, and flash elements, a web browser could have 100+ simultaneous TCP connections to a single URL. One major benefit of using multiple subdomains to call specific elements it that it allows content to load out of sequence. You might have seen sites that stall until a flash piece is loaded before displaying the rest of the page content. If the site would have called the flash piece from a subdomain, it would be loaded separately from the HTML and it wouldn’t stall the page load.
I have setup sites in this manner before to help overcome slow page-load times and saw a great increase. The site was and ecommerce site with lots of thumbnails, featured products, logos, etc. and it was nice to be able to call them from a separate hostname so loading the images didn’t slow down the site. Modern browsers will open 24 – 32 threads per hostname. You should take advantage of as many as possible.