I don't know anything about treadmills.
How do you know it is 30-40% off target speed?
Is the motor a common motor found in the expensive treadmills? Is it strong enough to keep up with the load? By load do you mean speed, weight of the person on it, or all of the above? I imagine heavier folks would put more friction on the walking/running belt, thus needing more power at the motor to maintain speed.
Years ago, I did disassemble a cheap one that had a hand crank to change the speed. The motor was a single speed motor and the crank was connected to something like a CVT. It was basically a pulley that could separate each half narrower or closer, changing where the drive belt rides in the drive pulley.
But like I said, I don't know about treadmills. I am just asking.