CPUs generally cap the upper limits of FPS. Unfortunately different games stress the CPU different amounts. The best way to know if you're CPU-limited is to monitor CPU and GPU utilization. If the CPU is at/near 100% utilization and the GPU isn't, you're CPU-limited for that game and settings.
Think of it like this, the CPU tells the GPU to render each frame (gives the GPU some basic metadata about what should be rendered), depending on the amount of detail in that frame, it could take the GPU a short or long time to render it. If your GPU can render that frame before your CPU can tell it the next frame to render (less graphically demanding game, lower in-game settings, or more CPU overhead), it has to wait (less than 100% utilization). If your GPU isn't done rendering that frame before the CPU feeds it the next one, the CPU has to wait.
***Don't get too hung up on utilization, it's not a perfect science. There is some [free] testing you can do, namely with in-game graphics settings. If you lower the in-game settings and FPS doesn't increase, that generally means you're CPU-limited.