/3/ - 3DCG

Is using shaders better than using post processing? I'm kind of new to game development and I'm not really sure about using post processing in a game because I get frame drops whenever I use it. I wanted to make a game with cell shading optimal for any device, PC or phone, but I'm not sure if it will perform the same because of the big change in hardware. So, what would be the best way to optimize processing for a better game performance? I thought about managing all visuals by multiple shaders but I'm not sure if it would be different from using post processing to get it done.
Also, what should I consider when developing for mobile?