Is this a symptom of a bad cell?

If it is cranking that long even at reduced speed, it does not have a 'bad cell' per se. With a bad cell, it would not hold a charge for long at all. More likely something like sulfation (just getting old and worn). The charging rate is not lying to you.

If it has removeable caps, do the following after it has set for a day or 2 after a good rive to get it charged, or after a good charge of any type: Remove the caps and connect a voltmeter to some bare steel nails with jumpers. Dip the pair of nails into adjacent cells and measure each cell voltage; on one of the end cells, you have to dip into one cell with one nail and contact the terminal with the other ot read the 6th cell. They should all be about the same, at around 2.1 volts. You can find any weak cells that way as well as using a hydrometer as above.