CPU may show as low because task manager shows you all cores together, which will never get near 100% if you have many cores, since the game can't use them all and can not use them all equally. Many Unity task have to happen on the main thread. GPU will typically not be at 100% as the game is usually CPU bound.
I will not do what that mod does. I don't want to disable AA on the weapon cam as it looks worse. The weapon cam may be going away at some point anyway. The increased blending they use also makes ugly motion blur, which the artists and I don't like.
In every case I've looked at, lowering settings will increase FPS regardless of what your peak FPS is. Be it 60 to 120 or 10 going up to 20. It sounds like your CPU is running very slow. Have you rebooted? What other programs are you running? Disk getting hammered by some OS update, backup running or a virus?