Best way to render irregular grid shapes

If you’ve played Candy Crush you’d see that the levels are all in a grid format but sometimes have odd shapes. I’m trying to replicate this functionality in my own game but the way I am doing it is causing the game to slow down on certain devices.

What I’m doing is creating a config file for each level, which is basically a 2D array of all the cells in the grid. If there is a blank space then I render nothing for that cell, otherwise I insert a background image for that specific cell, plus whatever item/obstacle is positioned at that cell.

The grid can be up to 15x15 but things start to lag at 11x11 (even on fairly powerful devices like the Galaxy Note 2). I’m assuming it’s because I’m rendering up to 255 cell background images, plus whatever items for the non-empty cells.

Is there a better way to do this?