If you have a jailbroken iPhone 3G that is sluggish after updating to iOS 4.1 you can try disabling the backgrounds. Because that isn’t configurable you have to hack your way in, here’s how:
0) install ericas utilities, vi and OpenSSH using Cydia
1) SSH in as root@youriphoneip (check your IP via Settings->Networking; use the default password “alpine”)
2) Navigate to /System/Library/CoreServices/SpringBoard.app
3) Convert the plist we need from binary to xml so you can edit it in a text editor:
plutil -convert xml1 N82AP.plist
4) Edit N82AP.plist file, and change this line:
5) Save the file, respring (maybe reboot).
Hope it helps.