To cut out large stumps, you make multiple passes. Take the stump down as far as you like, and then reposition to continue to take more off. Unless you have a large professional stump grinder, it is going to take a while. I hire out my stump grinding, and it is a lot less expensive than buying a stump grinder and doing it myself. I do own a large chipper, but that is because I have a lot to chip, and I work slowly.