Try switching off PhysX in AC4. It should increase your fps.
I was expecting the cpu usage to hit 100%.
Uninstalling Pando Media Booster, will help as it runs in the background P2P.
Is the cpu or gpu overclocked,
Also can you check the temperatures cpu and gpu when the fps drops.
On AC4, whilst playing on the settings recommended by GeForce Experience, I had an average 40FPS with some slight drops. I tried turning all the settings to the lowest possible (except resolution) and I still had 40FPS with some drops to 35. With the recommended settings + PhysX Particles on High the FPS drops were a lot more intense and it was not playable.
Throughout the whole time the CPU and GPU temps were between 50-65 Celsius. No higher than 68.