My system has Intel's flagship 6th gen Intel Core i7 6700k CPU, a high end GPU (Sapphire Nitro AMD Radeon R9 390 8GB GPU), and 16GB DDR4 2400Mhz HyperX system RAM and Kingston Fury HyperX SSD, and it sometimes gets poor fps in some areas of my parks like around the entrance but when I move the camera to other areas of the park, it shoots back up to 28-48fps.
The game is only in Alpha stage right now. You got to expect some imperfections. But how they've made the game, it's still a lot of fun to play it. The game is not optimised yet.
Cities Skylines requires a high speed CPU too. Yet with my PC specs, it can easily play the game on maxed out settings at over 60fps. Thank goodness the fps issue isn't everywhere in the game. In my experience, it's only in minimal areas of a park.