Is this a symptom of a bad cell?

Bad battery I agree. Take it in and have it load tested after a 12 + hour trickle charge or smart charge for an entire cycle. If you find a good auto place, they will charge your batt for you overnight and test it the next day. If the batt tests out fine then the starter or wiring is heating up and loosing efficiency.

I have cranked a 440 mopar with factory starter for 30 seconds straight and it has not slowed down more than 2-5% (estimated by ear).

I agree with pishta about the cranking time and lack of fuel in carb. We have a mild 440 / performer / 800 edl thunder. Usually after sitting overnight it takes 10 seconds of cranking, sitting for 2 weeks it takes nearly 1 minutes of cranking sometimes. Many speculations on the cause but overall reason it is - no fuel in bowl.

I just installed an economical airtex elec pump and I turn on the ig key, wait 10 seconds (till the carb fills with fuel), pump it once or twice (colder weather requires 3 pumps) and it immediately fires like it was fuel injected and fast idles on the choke nicely.

[ame="http://www.amazon.com/Airtex-E84070-Electric-Fuel-Pump/dp/B000DT7Y7U/ref=sr_1_5?s=automotive&ie=UTF8&qid=undefined&sr=1-5&keywords=airtex+universal+electric+fuel+pump"]Amazon.com: Airtex E84070 Electric Fuel Pump: Automotive[/ame]

.