Real world example: our project was taking almost 1 minute to compile via Ruby (all CSS).
Using sassc (libsass) that went to 5 seconds.
Development was where we saw real gains, different sections that would take 4-6 seconds to compile, went down to <500ms (our asset watcher is smart enough to compile only blocks of Sass files that are associated, not the entire site's Sass). Meaning that in the time it takes you to switch to your browser and refresh, the Sass would be compiled to CSS.