I do not know what voltage your motor draws and I do not know the battery capacity you have and I do not know how new those batteries are.
But in spite of all those unknowns it sounds to me that, that trolling motor is working very hard and that it is probably just fine.
Spot locking in hard winds takes a lot of energy and you cannot really compare performance by wind speed alone. 20 mph creates strong surface currents that are on top of wind speed.
and dependent on the wind direction there may not have been a lot of current when the wind speed was 10 to 15