Mine (2009 997.2) does that also... slow to crank on cold mornings.
2 yr old battery.

I'm not convinced its the battery, if the cranking is happening at the right frequency. If the starter is turning the engine over at the right rate, the battery has the juice, so its either the fuel, air or spark that is missing... or the engine would fire.

 

It seems like a classic HPFP symptom on the DFI engines. Video-tape the long crank and show it to the dealer, they should be able to troubleshoot it quickly and replace the HPFP if applicable.

Regards,

 

My car did the same thing and the dealer ended up changing the (don't know the name now) cord that goes from the batter in the front of the car to the back. Once they changed this part, it solved the problem.