Fps is all about the cpu. It takes the game files and pre-renders all that info into a frame according to IPC, clock speeds etc. It'll do so at 100% ability, the amount of fully pre-rendered frames in a second is your fps cap. This has nothing to do with usage. Usage is the amount of resources the cpu is required to use, ram or cores or Lcache etc to complete a frame. Doesn't make a difference if it's 20% of available resources or 80%, it's what gets used. There's a difference between how much a cpu uses and how much of a cpu is used.
Resolution and graphics detail settings, post processing affects etc has nothing to do with the cpu. That's all gpu. The gpu takes the pre-rendered frames sent from the cpu and finish renders them according to resolution and details. The amount of frames completed is fps on screen. This number can be same or lower than cpu fps, bet never higher, since a gpu can only work with whatever the cpu sends.
Basically ignore usage, it's a useless value for the most part. In your game settings, enable maximum settings preset. Then go back in and disable all pre-processing affects, like name tags or anything else that's bound to the cpu from game code. AA is a post-processing affect from gpu. This has the affect of giving the heaviest load possible on the gpu and the lightest load possible on the cpu, which equals maximum fps cap. Check fps count. Replay the same scene, but this time manually set all remaining details to disabled/lowest settings. This is lightest load on gpu, lightest load on cpu. Check the fps count.
If fps barely moved at all, then gpu detail settings are irrelevant, the cpu is capped at that level of output.
If fps makes pretty drastic changes, then the gpu cannot paint on screen the limit of frames sent from the cpu.
This'll give you a direction to start looking, whether at what's eating cpu ability or gpu outputs.
You can also get online and lookup tweeks. There's a lot of bs settings in the game and windows (Xbox DVR, game helper). Things like cloud detailing can greatly affect the gpu and yet the visual detailing is so small as to be generally no real difference between medium and ultra.