DIAGNOSIS If you've already ruled out a dull blade, this points to either a weak battery that can't deliver high amps anymore, or a problem with the tool's power regulation.
FIX First, test with a different DeWalt 20V battery, preferably a high-capacity 5Ah or 6Ah one. The smaller 1.5Ah or 2Ah batteries can struggle with the high draw of a circular saw. If a known-strong battery works fine, your original battery is failing. If even a strong battery bogs it down, the issue is in the saw.
IF THAT DIDN'T WORK The problem could be the brushes in the motor are worn out or sticking. On the DCS570B, the motor brushes are not user-replaceable without significant disassembly. This would require a motor service or replacement.
HEADS UP Also, make sure you're using the right type of blade. A framing blade with 24 teeth will rip through a 2x4. A fine-tooth finish blade (40+ teeth) will bog down because it's designed for slower, cleaner cuts in plywood.