In Cycles Preview (not rendering), GPU is 20 times faster than CPU, but only double in Rendering! In Cycles Preview (not rendering the image), my GPU is 20 times faster than my CPU, but only twice as fast in Rendering! The GPU and CPU speed, as stated, is compared in Preview mode, so if there was any thing that would weight it down in the actual render then it would affect both CPU/GPU basically the same (I think).

$500 GPU! - GTX 1060 (6GB), CPU - i7-6700

I tried tile size at 256 x 256.
CPUs work better when using tiles, escpecially smaller tiles. While GPUs work better when using bigger tiles or in the case of preview, 1 tile the size of the whole image. So when previewing it's just like one big tile, and that's why the GPU is much faster. and when rendering, it uses small tiles, and that's why the CPU performs better when rendering.