Typically optimization is one of the last stages of the development process. As such, the beta is probably not the best benchmark to see if your system can handle the final game. Most likely the beta will not run fantastic on the best of machines. In fact, Planet Coaster had several optimization updates post launch to ensure that it worked on all systems within the listed specs. Also, keep in mind that the list of minimum specs can and likely will fluctuate between now and launch as they test and decide how far they can optimize the code to work with lowest specs possible without compromising the game experience.
Not a response from a Frontier dev, but 2 cents from a software developer none-the-less.
This isn't to discourage you, quite the opposite. If the beta doesn't play the best for you, don't lose hope. Improvements will come and "could" improve your experience.