DirectX vs. GDI

I have post this message on VC++ before, remembered there was only one guy given the answer, and tell me something about this theme. But I think that's not enough.
What I asked before is "why the DirectX is faster than GDI", the guy with the kind heart told me that because there were less layer in DirectX than in GDI.
I agree with the standpoint, but I am sure there must be other things make the DirectX different from GDI. I am eagle to know what they are. Then when I am implementing some inferfaces for some specified systemes, I have had the key.
Any answers, include web address, articles and tips will be appreciated. Thanks!

A brief discussion on this can be found at this newsgroup posting. It may go over what you already know, but the points include:

• DirectX runs more of its time in kernel mode directly interacting with the hardware, where as GDI must go through an executive redirection (this is less of a problem on Win2000 and above business Windows versions).
• DirectX will try to optimize itself for available added hardware, whereas GDI continues (for the most part) to try to be hardware independent.
• Finally, DirectX uses a much more optimized drawing model to speed up the time from display/draw call to output.

I hope I have been able to provide some help to your question!