As IPhone 3g and IPod touch 2g has less ram and Processor as compared to 3Gs or 4G.so,when you update your iPod or iPhone to iOS 4.0/4.1/4.2.1,you will face some problems regarding speed like it way decrease the speed of your iPhone and your iDevice will seem to so slow and lethargic. Here is a Tip regarding that. This will help to increase the speed of your iDevice.
This is basically regarding to disable that features which you don’t use usually.
This will increase the speed as well as it will decrease the opening time of apps which in turn will increase the performance and speed of your gadget.
This tip will increase the features of device and opening time of applications
An iPhone 3g/iPod 2g on iOS 4.0/4.1/4.2.1
Step1: Go to settings>general>spotlight search
Step 2: Deselect all the options here. just as shown in the picture
This worked for me and I have significantly increased the speed of my ipod2g.Hope so it will work for you also .