you can speed up iphone 3g with following tips:-
First of all

, stop unnecessary Daemons from slowing our device. To do this, I have simply used a Cydia Tweak called Daemon Tools. It is found on the repo
repo.insanelyi.com. I have never found a problem with my iPhone after I have let this package take away some unnecessary Daemons, so hopefully you wont have a problem either.
Once Daemon tools is installed, I then installed Springtomize from the sinfuliphone repo. This app is great at removing features that will speed up your iphone a lot.
Once Springtomize is installed, go into it (Settings > Springtomize) and apply these tweaks
General - Disable Spotlight ON
Icons - Hide Shadow ON
Icons - Disable Jittering ON (Optional, but makes moving apps easier)
Dock - Hide Icons Reflection ON
Lockscreen - Slider - Custom Sting ON (leave the custom string empty) - In my experience this makes the lockscreen less laggy upon waking up from sleep
Folders - Remove Animation ON
App Switcher - Start Editing ON (Allows the multitasking Apps to automatically appear with an 'X' icon to remove from the multitasking tray)
Animations - Custom Duration ON (Have the slider bar almost all the way to the left) This option determines the speed of the animations that iOS makes, for example when moving through settings options.
Even though you are able to 'enable' homescreen wallpapers and multitasking in Springtomize, I ran into some difficulty enabling homescreen wallpapers via springtomize. After i enabled it in the springtomize settings area, I would not be able to choose what my homescreen wallpaper was (It stayed on the default waterdrop one). So if you want homescreen wallpapers and multitasking, it is important to select these features when jailbreaking via redsn0w.
