It's well know that GW2 is a game that rely too much on the CPU, the reasons vary, but it has a lot to do with multithreading. Basically: it sucks at it.
Our FX CPUS are notables for the poor single-threaded performance and, for that reason, they are literally the worst modern "high end" CPU you can have to play GW2 as the game cannot utilize all of our cores properly.
But why is that? A FX CPU with eight cores shares each CU (controller unit) with two of them: Each CU controls two cores, thus limiting it's performance.
BUT. There's a hope for us, mere mortals:
Most motherboards (I have a Gigabyte with a 970 chipset) have a option to force each CU to operate fully in conjunction with only a single core (or any other ratio we tell it to).
On mine it's called "ONE processor per CU". Selecting it disables 4 cores on my FX-8320 but makes Each CU control only one of the remaining cores.
After doing that, the CPU usage in-game boosted from ~35% up to ~80% and resolved part of the bottleneck it was causing on my GPU (R9 280X), wich again have boosted it's usage from ~40% to ~75%. My framerate (right after creating a new character) got about 10 FPS higher while doing the starting zone, and even hit 60 in some areas. And the sudden frame drops reduced a little too!
That being said, modern games or programs that can utilize all our cores properly will suffer from that, a lot. Streaming, video rendering, compacting files etc..
Why does it does not work with my four-cores FX CPU? GW2 can make use of four cores already, reducing it's amount will probably kill it's performance even more.
If you have a six core CPU, you can try another ratio. Your goal is to disable two cores and have your three CUs controlling four cores. I don't know if it's possible as I only have a eight core CPU.
I hope it helps someone. Have a nice day, Tyrians.