One of the most annoying things about S9y is the fact that the main stylesheet is a PHP script, which includes the bulk of the S9y codebase itself. Not only does this make retrieving the stylesheet a non-trivial operation, but it also means that the stylesheet itself cannot be cached. The reason why the S9y developers chose to do this is so that plugins can check to see if any styles are defined for particular classes, and if they are not provide reasonable defaults.

As an aside, a better method might be to call the main css file statically, and have each plugin provide its own static css file. It is trivial to check to see if 'my_plugin_name.css' exists in the current theme dir. All this could be done in the head block.

In the meantime, for any S9y theme authors, or even anyone who just wants to make their S9y installation just a little faster, here is a great fix:

1. In the current theme you are using, copy the style.css to main.css.
2. Change style.css to this: @import "/templates/your-theme-dir/main.css;.
3. View the text of http://yourblogdomain/serendipity.css. Ideally you should just have the @import directive.
4. If you don't that means that some plugins are defining their own default styles. To make sure the directives in your /main.css override them properly, you can just put the name of the class in a comment. For instance, I override the defaults of the FreeTag and searchQuery plugins in my style.css. So, my main css looks like this:

   @import "/templates/midibuffer/main.css";
   /*
   .serendipity_searchQuery {}
   .serendipity_freeTag {}
   */

There ya go. hopefully that will speed you up a little.