DIAGNOSIS Loss of power is rarely the motor itself. Nine times out of ten, it's a dull chain. The other time, it's a lubrication issue causing massive drag.
FIX Sharpen your chain. This is your number one task. A dull chain doesn't cut—it sands. The motor spends all its energy creating heat and dust instead of chips. Use the correct file size, maintain the original angles, and get every cutter to a sharp point. After sharpening, test it in a soft piece of wood without pushing. It should pull itself into the cut with little effort.
IF THAT DIDN'T WORK Check for excessive drag. 1) With the battery out, pull the chain around the bar by hand. It should move smoothly. If it's very stiff, your bar might be bent or the chain too tight. 2) Ensure oil is flowing abundantly. Run the saw and check for an oil line on your scrap wood. A dry chain creates incredible resistance.
HEADS UP Avoid using "rapid charge" batteries if you have them. Some users report that the high-output 20V Max batteries (like the PowerStack) can deliver more consistent power for tough cutting jobs than older or smaller capacity batteries. The tool is only as good as the sharpness of its edge and the juice you give it.