Use ScssPhp. to compile scss files on your wordpress install into a single lightweight CSS file. There is an included settings page for configuring directories, error reporting, compiling options, and auto enqueuing.
To keep the page load time to a minimum this plugin only runs the compiler when the scss files have been changed. All compiled files create or alter a matching css file in the chosen directory which remains even if this plugin is disabled so that your site never loses its styles and is always ready for user interaction.