Phone problem or Game problem?

Well I have an iPhone 3G, i know its not the best, fastest device but recently I feel that the game is laggy... I do some scout from my city and after 10-15 scouts or 2-3 minutes, the game significantly lags ( takes around 5 seconds for game to respond) and needing me to try again. I only have 1 device so I am not sure about this.

Does any other players have the same lagging issue??

i have the same issue with my iPhone 4

i have to close and restart the game and it work well again.

Are you running iOS4 or higher?
If so you should make sure to close down some of the apps running in the background. Though the older phone supports this, it does not handle the RAM usage very well. Double tap home and minus out some of the running applications. A half dozen running tasks is as many as you should ever leave.
Haypi should run much smoother when you do this.