Exactly this, you can't really fix the draw calls the game has when it's rendering so many player characters at once without multi-threading taking the brunt. Unfortunately this game (1.0) was designed around a time that it was common place to only use the main core and then a few for extra tasks. They didn't rewrite this in 2.0 (ARR), so we're stuck with a 10+ year handling of CPU logic from days of old. Not that modern gaming utilizes CPU cores very well anyways... I'm sure they've done some optimizations over the years but it's still not good enough for using anything above [Normal] setting in (Other): Character and Object Quantity.
The only real way to improve CPU-bound FPS by an ok amount here is to buy the 3D cache CPUs from AMD. Not everyone is willing to shell out that cash though, understandably.