When you first open your terraforming menu, you will see 4 icons:
1. Choose the brush and size it to your preference.
2. Click on the icon indicated by the green arrow. Y
3. You adjust the speed by clicking where it says step (red arrow).
4. Adjusting it up increases speed, adjusting it down decreases speed.
Be careful as to the speed you select as a number too high may give you undesired results. My suggestion is start low and gradually increase it until you find one that you are comfortable with.