There is a very simple fix for it.
Last week, Wall Street Journal reported that Apple was investigating the sub-par performance of iOS 4 on the iPhone 3G after user complaints flooded in. The most common complain was that the handset running iOS 4 was “sluggish” and would “burn through a charge” very quickly.
While the big boys begins their investigation, a reader over at Neowin may have stumbled upon a fix. And it’s quite a simple fix actually: all you have to do is turn off the spotlight search. To do this go to Settings > General > Home Button > Spotlight Search and turn it off.
Many users have responded positively, some reporting immediate performance gains that has fully or partial fixed the problem. While Apple determines the root cause of the problem and churns out an update, this could be the only solution for your struggling old friend.
Source: Neowin (via BGR).