DIAGNOSIS This is usually a blade problem, not a tool problem. You're likely using a dull blade, the wrong type of blade for the material, or a cheap, poorly-made aftermarket blade that can't transfer the oscillation efficiently.
FIX Swap in a brand new, sharp, high-quality blade that's specifically designed for the material you're cutting. For wood, use a bi-metal wood blade. For nails/wood, use a carbide grit blade. A fresh, proper blade makes a night-and-day difference in perceived power.
IF THAT DIDN'T WORK If it still bogs with a known-good blade, test with a different DeWalt 20V battery (5Ah or higher is best). A weak or failing battery can't deliver the needed amps. If the problem continues with a strong battery, the tool's motor or electronics may be failing.
HEADS UP Oscillating tool blades get dull way faster than people think, especially when cutting anything with adhesive (like underlayment) or abrasives. They're consumables. Trying to save money by running a blade into the ground just makes the tool seem broken.