There is nothing wrong. Use a big park and not a small one. On small ones I get 100fps too. For example load from the workshop this one: Six Flags New Orleans. The restoration of an abandoned park.
Tell me ur specs and ur fps. I get around 10-15. Unplayable and that park is not even really detailed.
Well it depends where I am in the park and what I'm doing, if you have a lot in the screen in one go, you're going to get lower fps than, say, when you're zoomed up close on something.
Here I have a pretty decent overview of the park with 25 fps (hard to see but fps is shown top left above the help button):
And then here I'm up close watching Batman and The Joker duking it out, with 34 fps:
But obviously that's not the whole story, cause I'm stationary.
Here I am riding Batman, at 23 fps:
Biggest FPS drops I get are when I'm sweeping my camera around on an overview of the park, I'll drop to 16-17 fps, but even then It's really smooth.
All these pictures are taken with everything turned on and set to ultra/high/very high whatever top option is (don't have tilt shift turned on obv, but that's it). GTX 1080, i7-6600k @ 4.5ghz, 3440x1440. Obviously they're decent specs, and I don't want this to be a case of "I can run it lol so u shud be able to". But seriously, what graphics settings are you using to get 10-15 fps?
And in a game like Planet Coaster, people using fps as some sort of benchmark really bothers me. For exploring a park you don't need more than 12-13 fps, it still moves really fluidly. There's reason for this and it's the way our eyes process imagery. When people want 60fps on games it's pretty much for fps(first person shooter) only, and even then it's for the skilled people, normal humans won't notice lower.
Every film you've ever watched (with the exception of like, the Hobbit or something) is shot in 24 fps, but that's even faster than it needs to be, it could be slower and we'd still be able to view films without discomfort. The only place where I think fps comes in to it in Planet Coaster is if we hope to get VR support, because your peripheral vision has higher sensitivity to the fps, if you were to ride roller coasters at 24 fps in VR you'd feel quite ill, but doing it on my computer (even with UW filling some of my peripheral) is totally fine. I think maybe another lower-limit would be the fps at which the the guests and staff are animated at - I think animation is usually done at 30fps, but I don't actually know what it is in Planet Coaster, I think I do notice some sluggish activity on peeps below like 17 fps, but my eyes aren't really sensitive to stuff like this.
I think the issue you're really experiencing is input lag, which can affect fps, even halting it completely, that's what will make the game "unplayable" for you, not the low fps itself.
And remember, guests change everything - left the game running whilst writing this, now at 11k guests with 14 fps, but it's still "fluid" for me.