Your battery running low will not effect your servo speed as you have a seperate power supply for that in the form of a BEC thats that conn that comes off the esc and goes to the RX. It is at a constent 5 volts. IF your battery could run down to 5 volts it would become landfill as it would be totally useless and cant be charged at that point.
The analog and digital servos are about the same speed and torque (you cannot see if one servo is .03 of a second faster then another) by eye.