DIAGNOSIS You're likely using the wrong battery. The DCF787 is a compact driver, designed for speed and finesse, not maximum torque. Pairing it with a small, low-capacity battery (like the 1.3Ah or 2Ah) will cause it to sag under load and deplete quickly.
FIX Get a bigger, higher-output battery. Use a DeWalt 5.0Ah 20V MAX battery or larger. The higher amp-hour (Ah) rating means more capacity, and the cells inside can deliver more sustained current, which gives the motor the power it needs under heavy load. You'll notice a huge difference in both runtime and perceived power.
IF THAT DIDN'T WORK If you're already using a 5.0Ah+ battery and it's still weak, check the speed setting. The DCF787 has a variable speed trigger, but no separate high/low gear. Make sure you're pulling the trigger fully for max RPM. Also, use impact-rated screws; standard wood screws have more friction.
HEADS UP This model isn't meant for lag bolts or heavy structural framing. If you're constantly driving long, thick screws, you've outgrown this compact driver and need a more powerful model like the DCF887.