Just took 3 full hours to charge my 6+ from dead to 100%.It's the amperage - all of them charge at 5V but higher amps charge faster. The "wall wart" with the S5 pushes at 2A (older phone wall warts were usually 1A). The USB port on the PC puts out 0.5A; your car charger is likely the same.
There's no reason the S5 cable (USB 3.0) would charge faster than a standard USB 2.0 cable with the same wall wart - none of the additional pins in 3.0 are for power. Unless samsung is retasking some of the 3.0 pins as a secondary power channel when used with their wallwart, but from the receiving end in the phone, seems like that would conflict with the USB 3.0 data functionality, IDK.
It will surely be dead by 8 or 9 am tomorrow at the latest I believe.