01 altima turns over slow when starting

My 2001 Altima has 194,000 miles on it and recently I have noticed that it seems to turn over slowly when starting the car. Initially, I had a battery that was just over 5 years old and with winter coming I decided to go ahead and get a new one and that would likely take care of it. After installing a new battery and monitoring for a while, I could tell that the problem is still present. Although it turns over slowly, it starts just fine. My concern is that this problem will declare itself on one of those -10 degree evenings when I am trying to leave work. I have thought about replacing the starter but I hate to throw money at something that may not be the problem. I measured less than one ohm resistance between battery negative and the engine block so I don't believe I have any grounding issue. Any ideas would be appreciated.