I was under the impression, that Blender would render faster, when the CPU is also checked.
However, with CPU and GPU checked, the render time is:
with denoise 00:15:24
no denoise: 00:06:52
Only GPU its:
with Denoise = 00:12.24
No Denoise = 00:04:30
Is this right?
Should using CPU and GPU be an advantage?
Nope, it’s significantly slower to check both. Just use GPU
Thats… why is that so? I mean i have 16 cores, why isn’t that utilised?
I have no idea why, I just know that it is 
hopefully someone else will come along and tell you the why
You didn’t test CPU only render speed.
A CPU is pitifully slow compared to a GPU for rendering. If you can render almost as fast with CPU only as with GPU only then combined render might be faster.
The render isn’t finished until the CPU finishes its part of the work. The GPU will have finished its work, taken a nap, gone on an ocean cruise, visited a brothel, and written a novel while we are still waiting for the CPU.
Even if it was done in a way that avoided this bottleneck … if the GPU has 291,617 Super Awesome Render Power Units and the CPU adds its 31 SARPU there would be no practical difference.
I man the CPU could start render the next frame while the GPU is rendering another.
Soe while The GPU handels like 90% of the frames… in that time the CPU might have rendered the 10% … and that would be a time saving.
That’s not how it works though- the CPU is being used either way, with GPU into rendering, it’s handling the data flow in/out of the GPU. With CPU/GPU, it’s handling that + the additional data flow + rendering + everything else (background processes), which ends up slowing down the data flow to the GPU. You’re bottlenecking your GPU by using both
You still didn’t test CPU only render speed. Try a CPU only render on your computer. You will see that the CPU has nothing to offer for rendering. 90% / 10%? Try 99.99% / 0.01%.
COULD? You have to render with the Blender that IS.
Plenty of people have over-thought this. Some smart methods were invented. None of them can overcome that the CPU simply has nothing to offer. It is so slow that even if its minuscule rendering ability was fully added to a GPU’s it would be adding a drop to an ocean.
Previous versions of Cycles were able to calculate per core, so CPU+GPU helped improve performance, but the CyclesX currently in use (I know since blender 3.0?) does not allow split calculation.
For that reason, if you set it to CPU+GPU, it will follow the processing speed of the slower CPU.
Even in CyclesX, CPU+GPU works if the GPU’s performance is similar to the CPU. (refer to video) 
Testing machine: MacBook Pro 2019, i9 2.4 GHz, macOS 12.3.1, Blender 3.1.2
Actually I indeed have testet CPU only. I know its slow, but this wasn’t in question.
I’m not sure what you’re looking for here - you know that GPU only rendering is faster, you’ve confirmed it on your own machine and other people have confirmed it as well. If you’re looking for an explanation of why GPU + CPU is slower, you have several of those already as well
The CPU is calculating what information the GPU needs for the next frame that the GPU is going to render. It already has a job, and is busy,
Yeah… well… its fine. I asked a question and got several answers.
I wanted to know if my observation is right and I wanted to know why and I got it.
Everything is alright.
Just added that indeed I have testet CPU only because it was said here that I haven’t… just because I havent posted the CPU Only results.
Anyway…
Hi,
I answered a question close to yours here, some time ago. Here’s the link.
Hope that helps you.