My app is frozen and slow in iPhone 4s

I'm developing an app which loads data from server in a UITableView and each cell is customized.

The problem is when I run this app on my iPhone 4s it is slow and frozen, but it works in other devices without having frozen or slowing. I have some images in my app used for cell background. How to solve this issue?

Why is is slow on iPhone 4s only?

Is there any way to solve this issue?