You're device has a RAM memory. Once the memory is filled up it lags your iphone/ipod down. There are applications in the appstore that cleans your RAM out and free's up space clearing up lag some. I use an app called "SYS-Activity" when I lag and it works well. Way better than restoring your iphone/ipod as that's a pain to constantly do. Also sometimes restoring will NOT fix your problem. Could be connection problems as well.

And simply shutting down your device will also free up some of your RAM memory but using an app that will do it in seconds is clearly easier than waiting 1-2 minutes to shut down your device and relaunch it.