Thanks for writing in!
X Theme doesn't uses much of images. Instead we take advantage of open source CSS frameworks bundled with internally developed libraries to design our visual elements. In my view combining images into sprite and then the effort required to position and implement the sprite images, won't improve the page load time much.
Instead you can improve the performance by implementing expires headers on your content served up from your host, utilizing a CDN, compressing media files (i.e. images and video) as much as possible without compromising the quality, gzipping server contents so they are compressed as they are sent, tweaking your .htaccess file as needed, and most importantly making sure your hosting is running smoothly without any issues.
We also have published documentation that you can refer to optimize page load time: