Here is what I think about Planet Coaster(hardware related), as long as Frontier doesn't add support for more than 4-6 cores it will lag no mater what, for me PC uses 43%(SysInternals Process Explorer) on my cpu Core i9 7900X.
My park has 1500 visitors, 38595 scenery objects, 9 rides and 2 coasters. It runs at 40-47 fps in daytime and 35-40 fps at night. At night gpu usage is at ~100%, daytime ~70%.
The lag all of you experience won't magically disappear because you upgrade your computer unless your hardware wasn't up to spec to begin with for PC.
I had a Core i7 5930K before, no change in PC performance after upgrading.
I play PC at 2560x1440. I have 2 GTX 980 Ti, I know PC doesn't support SLI.
Same goes for RAM, a ridiculous amount of it won't help believe me I know.
So unless Frontier does some amazing reprograming/optimization of PC(up to them) we will have to live with the fact that the game lags when there is a certain amount of visitors or when our parks has a huge amount of scenery in it combined with amount of visitors(amount of visitors seems affect the most).
*funny note* my park is based/built on one of the beta parks, my park is 397 years old[woah]
Hope I haven't offended anyone, this just my thoughts on OP's original post and PC's performance.