First of all iPhone 3G is not slow on iOS 4.2.1 though it is not as fast as it is on 3.1.3 (or 3.1.2). Now the question comes why do you want to update to a somewhat slower 4.2.1 when 3.1.3 is faster-
1. Most of the recent apps (and even games) are meant for >= iOS4.
2. Most of the jailbreak features like Winterboard that enables you to install themes and change wallpaper slows down iPhone 3G even on 3.1.3. On 4.2.1 you get the custom wallpaper option.
3. You even get option of enabling or disabling multitasking. True, that this feature alone slows down iphone 3G but you always have the option of disable it.
If you are on 3.1.2 (or 3.1.3) most llikely your baseband is 5.12.2 which is easily unlockable using Ultrasn0w.
1. Most of the recent apps (and even games) are meant for >= iOS4.
2. Most of the jailbreak features like Winterboard that enables you to install themes and change wallpaper slows down iPhone 3G even on 3.1.3. On 4.2.1 you get the custom wallpaper option.
3. You even get option of enabling or disabling multitasking. True, that this feature alone slows down iphone 3G but you always have the option of disable it.
If you are on 3.1.2 (or 3.1.3) most llikely your baseband is 5.12.2 which is easily unlockable using Ultrasn0w.