Diagnosis
The sandpaper is clogged (loaded) or the wrong type, or the battery is not delivering full power.
How to Fix It
First, change the sandpaper. Use sharp, new paper. For sanding paint or softwood, use coated aloxide paper, not cheap garnet paper which loads instantly. If using a dust collector, ensure the hose isn't kinked and causing drag on the pad.
If That Didn't Work
Test with a different, fully charged DeWalt 20V battery. A battery with weak cells will show full charge but sag under load. If it runs strong with a different battery, your first battery is failing.
Heads Up
This is a 20V MAX tool, not a corded beast. Manage your expectation for material removal rate. It's for finish work, not heavy stock removal.