IIRC the reason your FPS tanks is because FFXIV is CPU bound and uses very few cores. So if you open your task manager you will see 1 core at 100% and the rest very low, that gives you the 17% CPU usage or whatever. So yes, FFXIV could be improved to use all CPU cores and I think that would go a long way.

When you play at 4K it does become very GPU intensive, but the slowdowns you are talking about are because of the large quantity of players running around and your CPU calculating the positions for all of them. This is why the benchmark is smooth, there are no player movements to calculate.

It happens at Aetherytes because that's where a lot of people congregate. Thats also why we get instanced zones at the beginning of each expansion. Less people, less CPU strain, less FPS drops.