This plugin will add expires headers for various types of resources of website and also help to add optimized minification and merging of resources to have better performance and speed optimization.
Your theme is probably responsive which means it resizes the page to suit whatever device is displaying the page. If it's not responsive, you'll have to use a separate mobile plugin to render a page formatted for those visitors. The following plugins have been tested but YMMV depending on mobile client. You'll have to enable mobile browser support as well on the Advanced settings page.