I ran the A:M three teapots benchmark on my Core 2 Quad Q6600 machine.
At the stock speed of 2.4GHz it takes about 5:10 per frame
I have a utility to overclock the CPU.
At 2.8GHz it takes about 4:14 per frame, which is about 20% improvement for only a 16% increase in speed.
But at 3.0GHz the time goes up to 4:40 per frame.
So what goes wrong at that higher speed?