It uses less.
You won't need more page rendering time than normal direct reference css.
Sure! you look at the page source. You can see that less.js will insert a inline-style in the head.
Open firebug or other console, you'll see it.
less: parsed http://localhost/app.domain.com/static/style.less successfully. less: saving http://localhost/app.domain.com/static/style.less to cache. less: css for http://localhost/app.domain.com/static/style.less generated in 66ms less: css generated in 71ms
Above example is 71 ms slower than directly referencing css files.
Time consuming is mainly the file generation, and the insertion node can be ignored.