There are lots of ways to improve the performance of your website, some are easier than others to implement, and some will have a greater or lesser effect.
One potentially easy win is to limit the requests made from a single hostname. Most modern browsers will create up to six requests to a single host. If you are serving all of your resources from the same domain, e.g. http://www.mydomain.com then the browser will request the first six, wait for them to load, then start requesting the next six.
If you split your resources across multiple domains then this queuing will not occur. As a simple improvement, load your static content from content.mydomain.com (or similar). This means that your dynamic pages will load, and start pulling your static content very quickly, rather than waiting for all the requests to the dynamic pages to complete.
This splitting out also leads to other possible benefits. You can put your static resources onto a CDN, so the performance of your content.mydomain.com will be drastically better than if served from your own servers. It also allows for further tweaks, this domain can be configured to be cookieless (you’d use mydomaincontent.com in this case). That would save sending cookies for static requests, which you should never need.
This is a quick improvement that should be simple to setup. The major effort is in configuring the domains correctly, and planning your system to allow for these split domains. Once you’ve done that, you’ll increase performance for everyone using your site, without having to improve the code that drives the system.