You're missing the point.
For each frame generated for display, there are a set of tasks to complete. Some of them are done by the CPU, and some by the GPU.
The CPU has its own maximum frame rate - it can prepare only so many frames per second for the GPU to work on.
What's happening in your case is that the maximum frame rate your CPU supports is so low compared to the maximum frame rate that your GPU can handle that the GPU doesn't have to reach maximum clock speed to keep up.
Consider an analogy with cars. Car 'A' is a compact with a 90hp engine. If you press the accelerator to the floor, you might get up to about 75 miles per hour. Now put a 300hp sports car next to it. You don't need to floor it with the sports car to keep up with the compact.
Your CPU is the compact car - it's going as fast as it can. Your GPU, by comparison, is a sports car. It doesn't need to go as fast as it can to keep up - the GPU clock rate can remain at the middle speed and still render frames as fast as the CPU can prepare them. You would gain absolutely nothing by forcing the GPU to its highest clock speed.
Your only option to increase performance is to overclock or replace your CPU.