Many games like Skyrim or Saints Row 3 have performance detection included. Whenever I run it, the suggested graphics settings are "Ultra", "Maximum", etc., but when running the game on that settings, it lags like hell and is unplayable. Even medium graphics settings are often lagging.

Also, pages like Can you run it? are showing me I can play Metro 2033 on Maximum settings, but even minimum settings (everything disabled or lowest level) are lagging.

My graphics card is a nVidia GeForce 550M.

I've tried re-installing Windows from scratch, updating my drivers again and again but nothing.

Can you help me or at least tell me why performance detected settings and actual performance differ so much?