DIAGNOSIS: You're likely comparing a worn tool to a new one. The brushes in the motor wear down over time, reducing power. Also, the battery's age is a huge factor.
FIX: First, test with a brand new, fully charged DeWalt 20V MAX 5.0Ah or higher battery. Old or small-capacity batteries can't deliver the sustained current the tool needs for heavy work. If it's strong with a new battery, your old batteries are tired.
IF THAT DIDN'T WORK: If a new battery doesn't help, the motor brushes might be nearly gone. The DCF887 has user-replaceable brush cartridges. You can access them by popping off the plastic caps on the motor housing near the battery. Swap in new brushes (DeWalt part number 5140050-59).
HEADS UP: Carbon dust from worn brushes is conductive. When replacing them, blow out the motor area with compressed air to prevent the dust from causing a short.