This site is being deprecated.
Please see the official X‑Plane Support page for help.
There is no "frame rate cap" in X-Plane. In general, your fps will be limited by _either_ the GPU or CPU. We recommend aiming for 30 fps as a way to maximize rendering and graphics while also not impacting the flight model negatively.
I'm not sure why you would not see an improvement at lower rendering settings. If you're interested in going about it in a more methodical manner, check out the instructions here.
This site is no longer being actively maintained.