[cloudhead]

That's because the parser isn't optimized for compressed CSS.

By airing the file out a bit (just adding some double line-breaks), and passing -O2, it goes down to 177ms :)

[nex3]

This is real CSS handwritten by real people. It's not compressed, it's just not in a fully-expanded style. If your parser is slow for reasonably formatted CSS, then your parser is slow. The best case can be as fast as you want, but real-world use is what matters.

For what it's worth, -O2 brings the compile time for github.css down to about 1.5s.

[cloudhead]

The github CSS is compressed. It's fine to test worse-case scenario, but doesn't mean much.

[nex3]

No it's not. Look at it: it's one-selector-per-line, which is a very common style for handwritten CSS. If it were compressed, it would have no line breaks.