You still have the first generation of iPhone 3G and hate your iDevice since it still got iOS 3 and when you've updated it to iOS 4 it got terribly slow? well, this simple trick can help you make it run faster in iOS 4.
The cause that iOS 4 is slow in your iPhone 3G is because that iOS 4 has so much feature than iOS 3, Where a lot of it we can't use in iPhone 3G or even better, we never used it anyway such as sync mail and stuff like that.
The main problem lies in daemon, or we Windows user usually called it "Startup Entry". there's lot of addition in iOS 4 that'll make your iPhone 3G got really limited resources. In Windows environment we can just run 'msconfig' and uncheck any startup that we didn't want.
Can we use that method in iPhone? sure we can, although it not that simple. In Fact this trick can be used in other type of iPhone, but lets focus in iPhone 3G.
I assume here you've already jailbreak your iPhone since it already didn't have any warranty after all.
Here some simple trick that I found :
Open Cydia and go to 'Manage' > 'Sources' and add new sources with link
after you add it, search and install App called "Disable Daemon" then restart.
You'll notice some real speed improvement now.
If you want to step further, There's some article that you can read in how to remove these daemon in order to improve the performance :
Hope this can help your problem with the speed.
Have A Nice Day..