As told, the game is poorly designed. Very hard to "optimize".the transitions between scenes are REALLLLLLY slow, is there any fix?
So, you need more "power" to run the game smoothly.
Did you try using an SSD drive? Put your game in an SSD. It will help a lot. If you are using a basic HDD, you will have a hard time.
Then it's better CPU. More & better RAM. Better graphic card etc...
But beyond that, not much you can do. The game loads a truckload of data each time and then hides them from you and show only what's needed in the scene.
It's poor design, and it was bloated with content through the updates.