Apple discontinued support for the iPhone 2G and iPod Touch 1G with the release of iOS 4.0 and both devices can have iOS 4.2.1 but lack of several core features. Even though iPhone 3G owners can enjoy the iOS 4+, they experienced a lot of of problems after the update and the obvious one is the iOS being completely slow.
This beleaguered issue outraged many iPhone users and it was fixed with the release of iOS 4.2.1 even though the sluggish performance is still noticeable. This has pushed iPhone users to downgrade their devices to iOS 3.1.3.
The WhiteD00r is basically a modded version of OS 3.1.3 that looked and performed exactly like iOS 4. The developers behind the WhiteD00r project have recently updated modded iOS 3.1.3 with custom firmware file which contributes a lot of performance tweaks. You can actually watch the official video of WhiteD00r which compares iPhone running on both WhiteD00r and the stock version of iOS 4.2.1.