The report claims 15 minutes to the police department. Google shows 14 minutes. Based on the route taken, this would be an average speed of 18 to 26 MPH. Much of the route is a state highway, and part of it a US highway (albeit on city streets). The longest stretch is on a state highway with a wide median, which I'd expect to have a speed limit of 40 or 45. So Google may take into account average number of stoplights. A police car at close to 8 pm, could simply flip its siren and go through a stop light.
The consolidated video is not in time order. So it is possible that the time stamps are not accurate, or maybe the interior videos are not in sequence.
The report of the first police officer on the scene says he took Zimmerman into custody, and then observed the 2nd officer try to revive Martin, followed by other officers and the paramedics "who
began to give aid to [Martin]".
The officer then reports that he placed Zimmerman in the back of his patrol car, where the SFD gave first aid, and he overheard Zimmerman say that he had called for help.
The second police officer reported that when he arrived, Zimmerman was in custody, and that he then tried to get a response from Martin, and that he and another officer did CPR, and then another officer, and finally the paramedics, who pronounced Martin dead at 7:30. We don't know what the basis for this time was.
CNN's timeline puts these back to back, and in essence says that the paramedic who pronounced Martin dead, then came over and checked Zimmerman over. In Houston, when they send an ambulance, they also send a fire truck. And there is no reason that in Sanford they only sent one ambulance.
The officer who had Zimmerman in custody, only mentions the beginning of aid to Martin; and the officer who first performed CPR just mentions that the other officer had Zimmerman in custody.
So you can easily get another 15 minutes of treatment.