GHz. for dual core CPU's between the m3, i5 and i7's is the m3 is the least powerful, followed by the i5, then i7.
On the other end is the quad core i7 which is an option on the 15" 2016 which can outperform faster "GHz." Dual Core CPU's that in tasks that were optimized for multi-core use. In many cases the quad cores are significantly better for video editing, print design, hi-res photo retouching, 3D and animation, and multi-track music recording and mixing. For day-to-day tasks like web-surfing you would see little or no benefit, in fact you would decrease battery life powering the CPU. Most of the newer mac laptops sacrifice CPU power for longer battery life.