kernel performance: 3.16 vs. 4.9

[Matthias Bodenbinder <matthias@bodenbinder.de>]

Hi,

I have upgraded my PC to newest chipset and CPU: Kaby Lake, Z270 with i7-7700K.

Out of curiosity I did a kernel benchmark. Comparing darktable performance with kernel 3.16.0-4-amd64 and 4.9.0-1-amd64. I use the following command to run darktable:
darktable-cli test.CR2 test.jpg --core -d perf -d opencl

The results are surprising for me. Kernel 4.9 very much outperformance kernel 3.16. Here are the results with and without opencl (using a Geforce GTX750TI):

			kernel 3.16	kernel 4.9
with opencl		16 s		9 s
without opencl		120 s		23 s

Without opencl, that is with pure CPU performance, the difference is a factor of 5! 

Why is that? What am I missing? I can hardly believe that kernel 4.9 is so much faster. 

Matthias