If you are not familiar with CSS sprites that you should start doing it now. CSS sprites are the best way to improve your plugin speed very easily.
If you ever look into the code of WordPress itself you will find that it’s using a lot of CSS sprites when displaying images and icons.
What are CSS Sprites?
Here a good video that explain CSS sprites a very good way
Reducing loading time and the number of requests can improve the overall plugin speed.
So next time you develop a plugin use CSS sprites your plugin user will love you more.
You should know that should try to use CSS sprites inside your plugin both in the front end and the administration sections.