And you're manually shifting or letting the car do the work? I would imagine the car doesn't get past 2nd gear to go 15mph. I'd take it to get checked out if it's doing it on its own. If you're manually shifting, I'd just tell you to stop because it seems unnecessary, imo10% throttle. I park in a parking garage and I am normally going <15MPH for a few minutes until I get out on the street. This is when it's the most noticeable. Harder throttle seems to be "better".
Don't know if actual temperature is a factor. California "cold" is like 50*F