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.
This plugin is available for download to be used on your WordPress self-hosted installation.