If it is maxed out (red line), you are not CPU bottlenecked. You are VRAM bottlenecked. The only fix is a bigger graphics card—not a faster CPU.
Here is what happens on the timeline when you enable MFR: If it is maxed out (red line), you are not CPU bottlenecked
You can manually control how hard After Effects pushes your hardware. If it is maxed out (red line), you are not CPU bottlenecked