I've tested my iPhone 4 with a stock and jailbroken 3GS and in both cases my 4 opened folders and brought up the app switcher menu slower. Outside of what is already installed in Cydia I installed appbackup, infinifolders (the folders opened slower before installing this), OpenSSH, Quickreply, and SBSettings. Has anyone seen a slowdown with their iPhone 4? I'm wondering what could be causing it to be slower than a 3GS?