This game only uses Directx11. THAT is the big problem.
With Directx11, it only efficiently uses one CPU core max for important CPU work.
If it utilized Directx12, the game would be far more optimized and smoother.
I have a high end Intel Core i7-6700k (4 cores, 8 threads, base speed 4Ghz turbo boost to 4.2Ghz, this is stock speed, and it can be overclocked). And can tell that from Windows 10 Task Manager and MSI Afterburner, that this game is not using my CPU to it's full potential. I see one core at like 90% utilization but the rest are like 20-30's utilization.
On my latest big park with 22 rides, and 7k park population with moderate scenery, in 4k mode with graphics on the higher settings with water quality on low and shadow quality on low, I'm getting 17-23fps. And it stays that way even when I change the graphics setting to Low everything.
So it's not a GPU problem. It's the problem and limitation which Directx11 has.
The core issue is the limitations of Directx11.
Directx12 doesn't have that limitation, as it introduces the possibility of efficiently using more than just one core, which Directx11 does.
However many people are still on Windows 7/8/8/1. Which is why Frontier put less importance on Directx12 or making a dedicated version for Mac.
What you want is at least a quad core CPU with a high clock speed. The higher the better. 6 core and 8 core CPU's can help with the extra processing, but only as far as Directx11 allows it, since this game is built around Directx11.
Intel is possibly releasing the Intel Core i7-9900k September this year maybe later maybe even in 2019. Many common sources rumour it to have 8 cores 16 threads and up to 5Ghz clock speed.