This plugin is great for performance and page speed - it adds only 1 css file and 1 js file to the page. You can unhook the css file to add your own styles in your theme's css, which leaves only the minified js file added to the page.
It does what it says - adds a lightbox to linked images on your website. There's no configuration necessary to make it work. Just make sure your images are linked, and your gallery shortcodes have a link="file" in them.
Very pleased with plugin!