EDIT 28-Sep-2011 : Do not take this as a definitive fix. I need to hard reset my phone every day with 4.2.1 – The reasons why I wanted 4.2.1 were very questionable (tilt-shift filter capacity in instagram!), therefore a revert to 3.1.3 is planned, and so is the purchase of a Windows Phone 7 model.
I've recently installed iOS 4.2.1 over my jailbreaked 3.1.3 because of some apps I needed. I was struck by the very poor responsiveness of the device. I recall it being way faster when I bought it two years ago.
I searched for resources on the net and found inspiration here: http://appletoolbox.com/2010/11/iphone-slow-after-ios-4-2-1-update-fixes/
I remember that with Location Services disabled I was avoiding a lot of hassle and delays in some social apps. Checking the advices provided, I decided to disable the following services:
– Spotlight Search
– Location Services
For smooth user experience reasons, I also disabled the Auto join WiFi network prompt, which always jumps on your face in the most inappropriate moments.
I followed these changes by two consecutive hard resets.
Guess what? Someone is happy with his rejuvenated iPhone 3G!