The only thing ported over that causes a slowdown is the window double buffering. And that's only to make up for Windows' crappy windowing system (on XP at least). Most everything else (graphics, text, networking) either uses the system libraries or a faster Apple library.baggage of forcing an independent rendering library that cause each operation to slow down.
baggage of forcing an UI that breaks window guideline and create Interface lag when operating on the UI, such as maximize.
The way safari is ported, causes both rendering and application speed to slow down and can not compete with opera at speed.
please listen to what windows users are saying before porting OSX users opinions. safari for windows is a total different beast than safari for OSX.