A lot of users have been complaining about their iPhone 3G getting slower in terms of performance after the update to OS 3.0 . Boot and shutdown take longer now than before and most apps also launch slowly. Safari has also been lagging a lot. This has been confirmed by users who haven’t Jail broken their iPhones 3G. Some have even reported a performance fix via jail breaking the firmware.
There’s no official word on these issues that some OS 3.0 upgraders are facing. The upgrade to OS 2.0 was slow and buggy for most as well which was fixed by update 2.0.1 We might be getting a 3.0.1 update soon as well.