Performance tips on grid base game

Putting 40 x 27 blocks of images on a game board should not be giving any performance issues. So without knowing what's causing the slowdown it's hard to tell what to improve.

I have edited my question with more detail. Rendering cost affect the performance of game. If I assign none sprite to wall then it does not create any performance problem or fps loss but if I attach sprite to it then it gives loss in fps in mobile devices. Though there is no problem of fps in pc game.