All apps use a standard framework for the iPhone, so the graphics are handled by the OS.
Apps are "faster" for many reasons:
1. It doesn't have anything running in the background.
2. Most apps are programmed on one language (does the iPhone support other then C++/C/ObjectC?)
3. It's running at half the resolution of the N900.
4. It doesn't use GTK + X server + a window manager to handle stuff.
And so on...
Jailbreak an iPhone and make it multitask, then ask again.

P.S, interesting use of tags...