I can confirm what Skywaterbirdx16m said about the CPUs. The bottleneck seems to be (like in the good old Havok engine days back in 2005-2009) the speed of your CPU. But this problem is very dodgy for developers, as the CPU speed is (partially) also depending on the peripherals which aren't mentioned prominently in a DxDiag. We had that back in 2008 while modding some games, like X3 TerranConflict. The problem is that data input is handled in cycles, which was back in 2008 a big problem, as there were no multi core CPUs, nor optimasations for those. The max you had was a double core. So input from peripherals, like mouse, keyboard, joystick, headset etc., which was connected through USB, was messing up the cycles, cluttering the whole CPU performance. Especially those "gamer" keyboards were horrible ressource hogs, as people tended to keep track of certain ingame informations, or their playlists on the LCD displays integrated on these keyboards. Even some of some cheaper USB sticks had an impact, as all operations had to pass the Bus. The problem is and was that operations were directed to a single core so the game itself came almost to a screeching halt when high polygon objects had to be displayed.
This was almost ten years ago with Dx9. Today there are better solutions, but depends on how the ressources are handled by the game engine itself. As the Cobra engiine is definately not that old, the devs will probably have taken care of cluttered cycles, but it still depends. My rig is a bit older, 2 years, but it's still able to handle 4k+ peeps in a park. It's a core i7 with 4.4 GHz quad core with a NVidia 960. I just can't fill the whole area of a challenge or sandbox park. But to be honest, that's a titanic quest which only the most dedicated Planet Coaster players will master

and i tend to be satisfied and out of ideas when roughly 1/3 to 1/2 of the area is covered. All in all, i think you do need to be a bit aware of all those gadgets running as peripherals.
By the way, one of the problems can be the onBoard sound. Sometimes a good old soundcard like a soundblaster with proper drivers can help a system. The onBoard soundchips are rarely getting an update and their quality is fairly low, as they're often manufactured by companies like Realtek. We had already posts mentioning that they can't hear their custom music, while the native frontier soundtrack still works and the game does not crash. I doubt that this is a problem of the game engine, but a problem of drivers. Anyway , if you like to play giant parks with lots of people inside, i'd not only have an eye on the CPU and GPU, but also on the connected components, like DVD/BlueRay device, plugged in USB peripherals like keyboard, mouse and eventually your router/modem. Also, the steam can be problematic, if there are steam cloud services are trying to synchronize your system. Steam itself is since the summer update not terminated correctly anymore. On some systems windows needs to close steam by force when powering down. Try to keep steam offline as long as you play bigger parks. Helps a lot sometimes.