Performance of shaders

[.:HyPeX:.]

Well ive struggled with text quality for a while now and ive found something weird: Shaders increase text quality.

Now, what i want to know is how big the performance impact is. And for those wondering, do a simple try, draw a text into a render target and then place the render target as a texture of the simple shader example. Then proceed to draw both the render and the shader, and the shader will have better quality. (In fact increasing the shader sub pixel increases the quality even a bit more, tho is barely noticeable).

So anyone has an idea of how bad the performance impact is of doing this?