There may be several reasons for an iPhone to run slow. An iPhone runs slow if there are lots of apps installed, lots of text messages in the SMS app, too many photos, songs and other files, huge list of history and cookies stored in Safari, etc. Well, most of the speed problems are related to iPhone 2G and 3G as they have only 128MB memory unlike the 3GS which has 256MB.
Firstly, delete text messages in your SMS app if there are lots of them. Also, clear the internet browsing history and cache. These two things slow down an iPhone to a great extent. Well, that’s what I observed on my iPhone. Safari browser had extremely slowed down and I could hardly scroll.
Next, download MemTool from Cydia. This app allows you to kill processes and free up memory which in turn speeds up your phone. Simply click on the ‘Clean’ button and some memory should be freed. You can also view the processes and check out which process is consuming lot of memory and CPU.
Thirdly, you can try deleting some unwanted photos, music and apps. If you still feel your phone is slow then the next best thing to do is restore the firmware. You can restore a firmware by pressing Shift + Restore in iTunes.