Why shaders affect the game performance so much?

I am developing an FPS multiplayer game for android platform. I have the initial build ready for desktop and it runs perfectly in my laptop but when I made an build for android and tried to run it on my Lenovo Ideatab A1000G but the frame rate dropped to nearly 2 fps. Later I read somewhere(probably some post in the forum) that I should use mobile shaders for games aimed for mobile platforms. When I changed all the shaders to mobile shaders the frame rate bumped up to 15-20 fps. But how can I get a good rate of 60fps.

Shaders are the programs that run on your graphics card to draw output to the screen. Every game has them (or else you wouldn’t see anything). And, like every other bit of code in your game, if they’re not optimised, they can be a bottleneck.
There’s nothing special about “mobile” shaders, other than they are simpler versions of shaders that use less effects (e.g. they might use less textures, or not calculate shadows) in order to run faster on less-powerful GPUs.

To get a good framerate, you need to understand what your bottleneck is - use the profiler (or, at the very least, the stats window in game mode) and post the results here.