Well, here is the thing. This has nothing to do with Hyperthreading or your system being too slow. Instead it has everything to do with your graphics card memory. Even if you have something like a GeForce RTX 3070 or 4060 you can experience this issue, because these cards 'only' have 8GB of video memory available. Whenever the game produces scenes that go over this threshold (e.g. too many objects come into view, or bad memory handling) the game has to additionally enlist your slower system memory (commonly known as RAM) to get the job done. That's when you see these seemingly inexplicable slowdowns. As soon as the memory usage falls below the 8GB the game returns to higher frame rates, because your graphics card can handle the task solo again.

Go to your Graphics Settings in game and change the Texture Quality from Very High to High. That's the single most demanding thing when it comes to memory. Depending on your system, you might have to go even a bit lower or you could try to additionally reduce Level of Detail as well as Crowd and Traffic Density. Remember, you want to give your graphics card more leeway so it doesn't run out of memory.

That's it ... If this doesn't solve your issue, your system has some other problem, I'm afraid.