This is a really nice package and you need to do it carefully, but we try on several devices including iPhone 4s and this is working great.
Go to :
Jailbreak your iPhone.
After go into Cydia Apps
Install the Package Accelerate
When done goto : Settings, Accelerate
Choose your Speed !
Use at your own risk. To be honest if you are not familiar with IT or have basic knowledge in iPhone I will not suggest to try it by yourself. Ask a friend !