You're applying too much pressure to the Smart Drive paddles. Ease up.
The Smart Drive system has a variable speed transmission. The speed is directly controlled by how hard you press the paddles on the handlebar. For better control on tricky terrain or when you want a slower pace, apply less pressure.
If that didn't work, check for other issues causing poor mowing results. A mower moving too fast for conditions can leave behind cut grass. Also ensure your cutting height levers are all set equally and the grass bag is empty.