There is always room for improvement. Devs know it too, they themselves said there are some things they are working on and are not ideal. They also promised they will optimize the game even more in the future. Saying this game is very well optimized is wrong then, because if it was well optimized, devs wouldn´t have to work on this side of the game.
People saying it´s game with limiteless creativity, that´s right, we can expect the game to become laggy at some point, sure. But there simply is something wrong. My park doesn´t have too many buildings and scenery, yet it lags ( and it gets inbearable when I hit 10-12 k people). When park closed, it keeps the same FPS and when deleting buildings, it doesn´t recover too. So I would say there is something wrong then. Because if I empty my park again, I would expect fps to go back to normal. When I get PC installed again, I might upload my park so devs can take a look at it.
I also stand by that UI is not optimized. item lists take some time to load sometimes (parkitect had the same issue and they addressed it). Frontier did great job though, just hate people saying it´s all fine, because it isn´t yet. And if we won´t tell them, then it will be harder for them to improve things. I´m hoping for UI getting some love with this update and that it will finally get reorganized and faster. I would love to at least hear some more info about their CPU scaling thing, because I´m curious how it works and depending on that, I think it might help to get game running smoother for some people.