Because those performance issues are only present with specific games, such as this one.
CPU: An i5 7600k 3.8GHz
GPU: an MSI RX 470 Gaming X 4GB
8GB RAM
The only criticism I'd give for your rig is the RAM... Assuming it's a single stick, double it to 16.
We're definitely into the point of 8GB being a bare minimum for RAM, but I'm holding off until I can afford 16GB DDR4 plus the mobo and processor. Tad expensive and will take time, but it'll be worth it.
As for your issues with the game, I'm going to go out on a limb and say that it's an issue regarding Unity itself.
As an engine, Unity's biggest "selling point" is the price for the Personal Edition - Free, which is good for new devs and indie titles. However, it's also pretty notorious for being problematic for PC games, but that has been (very slowly) becoming less of an issue in the thirteen years it's been around for (updates to Unity have been making improvements).
One person's rig can be a "top-of-the-line model" that cost £2000-£3000 and have issues with Unity games, whilst the next person could have an equal or lesser build that doesn't.
A common issue for some games made on Unity (that has been noted here by some users too) is the loading times/transitions from one location/map/room to another.
My most recent Unity title (using the Professional Edition) is Battletech. Aside from playing a lot better than on my largely obsolete old PC, it does sometimes have loading times between entering and exiting a mission that can be randomly longer than normal, even on the exact same map that had been played on in the previous mission.
Even issues in the time taken from transitioning from one store category to another can randomly "hiccup" too.
One solution had been to let the game have no more than 10 saves in the save directory at any one time, which can mean doing a lot of deleting of autosaves (separate autosaves for every out of combat event, mission start and end and others). Even on the most recent version, I still do that almost religiously, just to make sure that it doesn't become a problem.
Outside of Battletech, I've seen it in the older Unity title of Wasteland 2. Love the game to death, but have to be careful with saves - again no more than 10, and that's an older version of Unity than Battletech uses.
