I think My iPhone4 is a bit slow...?

It is iPhone 4. iOS7 is updated. It is 16GB and I still can use 901MB. (I think I used a lot of them) Asphalt 8 is so lagging in 3G. (It works pretty well when I'm using my house's internet but it's so lagging in other public internet) When I went outside, 3G works so slowly even though the connection bars are full. (sometimes it works normally) I used it over 2 years. Sometimes phone call doesn't work. (It is going so lagging and it just turns off before the signal sound) Please help me.