My 3GS 6.0.1 extremly slow, need to factory reset without loosing Unlock

So just weeks ago my iphone got extremly slow. unlocking takes some 10 seconds! answering phone is so hard! iphone 3gs old bootrom ios 6.0.1 jailbroken and unlocked with Ultrasn0w. there should be a software problem so I want to restore it to a clean iphone without loosing jailbreak and Unlock and then see if it is slow or not. thanks