He a little explanation about GPU vs CPU rendering in Blender:
GPUs are generally faster than CPUs, if you spend the same amount of money for them, so if you spend 500 dollars on a GPU and on a CPU, the GPU will be several times faster in rendering.
CPUs are made to run code, not to crunch numbers. They CAN, but they are more inefficient doing it. GPUs on the other hand (using OpenCL from AMD and such and CUDA / Optix (proprietary to Nvidia) can't run normal app code, but are perfectly suited for number crunching. That is for tasks that can be massively parallelized, as GPUs have thousands of tiny little cores (Nvidia calls them CUDA cores on their GPUs).
If you consult Blender Benchmark, you will notice in order to match the performance of a Nvida RTX 3090 GPU that will nominally cost you around 700 dollars, you will need to spend several THOUSAND dollars for a CPU that can best that.
(these prices are not accounting for the insane inflation of cost that Covid 19 has wrought on electronic components)
Now notice that even a 64 Core AMD Threadripper (a insanely powerful) CPU can't keep up with a Nvidia RTX 3090 GPU, while still costing a multiple of the GPU.
Also note that Blender Benchmark is unbiased, it is a real life benchmark that uses Blender to just render scenes, it is the same as if you rendered these scenes yourself.
So you could say that CPUs and GPUs are just different, they both have specializations and best work together, CPUs for running code and general computing and GPUs for either displaying graphics or raw super fast number crunching of calculation loads that can be massively parallelized.
In Blender, if you want to render scenes really fast, you will need to go with a GPU, right now as of this writing (July 2021), looking at Blender Benchmark https://opendata.blender.org/ the GPU to buy would be a Nvidia RTX 3080 or 3090 or even 2 or 3 of them, since they can be used together. There is no CPU currently that can best this kind of raw calculation performance by for example 2 RTX GPUs.
Power consumption is also interesting: Your typical powerful 12 core AMD CPU for example will draw up to 100 to 120 watts of power. A single Nvidia RTX 3090 will use up to 350 to 400 watts of power. As they say: there is no free lunch.
It is also noteworthy that it seems that CUDA (which allows calculations to be done on the shader cores of the GPU, instead of using it to display game graphics) is now being replaced by the much faster OPTIX. It seems that the huge success of Nvidia cards in the render field was due to Nvidia highly optimizing CUDA and now OPTIX for this. Nvidia had an obvious incentive to do this since CUDA is proprietary and can only be used with Nvidia GPUs.
When using OPTIX on RTX cards, the new tensor cores can be utilized and sometimes the speed can be double of what CUDA would deliver on the same card, see this article here:
Blender 2.81 Benchmarks On 19 NVIDIA Graphics Cards - RTX OptiX Rendering Performance Is Incredible
This might sound biased towards Nvidia, but it is actually just stating facts and AMD has made great leaps in the last GPU they released, closing the gap to CUDA performance, but not OPTIX performance yet, it is expected that they will use a faster replacement for OpenCL in the future, so we can all be hopeful for more competition in the market.