I think my iPhone 4 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)

It's just an older phone and iOS 7 drags the iPhone 4 down.