This post will mention some of the things you can do to decrease the page load time in a Java-based web application with regards to Yahoo!'s list of best practices for speeding up your web site (http://developer.yahoo.com/performance/rules.html).
Minimize HTTP Requests
Regarding reducing the number of requests for images, CSS sprites can be used. Use, for instance, SpriteMe (http://spriteme.org) or CSS Sprite Generator (http://spritegen.website-performance.org) to create the sprite. I use CSS Sprite Generator to create a starting point, and then modify and maintain the sprite by hand (GIMP). Be aware of the placement of the images in the sprite so that adjacent images don't show up in the wrong places in your page.
Add an Expires or a Cache-Control Header
By default, wro4j sets the Expires-header to be ten years in the future for content served through the wro4j filter. This tells the client browser that the content won't change for ten years, so it shouldn't bother to ask for it again before then. Be aware that the browser won't even bother to ask if the content has changed, so, unless the content is removed from the browser's cache, it will use it's cached version for ten years. This means that if you modify one of the files served by wro4j, but don't change the reference to the group containing the file, the clients won't get the updated file for ten years.
Gzip is a compression method that generally reduces the size of your files about 70%. Once again, wro4j does the job for you. By setting the filter's gzipResources init parameter to TRUE, wro4j will gzip your CSS and JS resources. Images shouldn't be gzipped because they are compressed already.