"Updating shaders" render problem

I have a very simple scene, with very extremely simple materials without glossy or bump or anything else.

Yet I cannot render an animation fast, because it has to show this “updating shaders” every single frame I render, every time. And it takes about half of the render time. Even if it is a matter of few seconds or miliseconds, it quickly adds up to minutes, during an animation.

I want to know how to get rid of the “updating shaders” process during render, because it only does it when my scene gets complex enough, and it doesn’t have to be that complex for it to show this annoying “updating shaders” message.

Please help me!

BTW. I am using GPU rendering.