As part of the build process, automatically merge all css files for a theme into everything_unpacked.css, then pack it with yui compressor into everything.css
Defaults to false for easier development.
Set this to true to load everything.css
You can also override the setting via the css_fast_load parameter, just like js_fast_load.
How much faster is this? Without it, you have to load main.css, which then loads other css files (these separate files exist so that it's easier to understand what they do). You load 9 css files. The first 6 take about 15 ms, the last 2 take about 31 ms.
With the packed version, it takes 15 ms for all of them.
This will be specially noticable if you have a bad network.