opencv function implementation

opencv function implementation

I wonder how does opencv do operations on Matrices. For example, when I
write code for
cv::add (Mat mat1, Mat mat2, Mat &result)
using two for loops, it takes around 120-130 ms for 1000x750 image. But
using opencv add function it takes 6-7 ms. Does anyone know what is their
trick? I want to learn it to be able to write functions that opencv
doesn't have.
I have searched inside opencv and find this two .cpp files(first, second)
but I dont know if I'm looking at correct place.
I just want to know how to use this power. Could somebody help me?
Thanks,