Turn mower off and remove its disabling device. Put on heavy gloves. Inspect drive and rear wheels along with mowing blade for stuck or accumulated mulch, vines, rope, toys, small branches etc. Clean if needed then make sure wheels rotate freely and let the mower mow for a week to see is mowing duration improves. A health battery should give 1.5-2 hours of mowing if set on 120 minutes mowing duration.
If Step 1 was not the issue, make sure the blade is sharp, centered and not wobbly. If dull or wobbly replace mow blade and monitor mow times.