There is no way to calculate this from the given information.
For an estimate you need the rpm <-> torque curve of the builtin electric motor. This is usually only taken at a constant rpm, so I does not tell you much about the inertia from the rotating pieces, such as the gears, in the servo.
Additionally a 'digital' servo implies some kind of control loop to accelerate and decelerate the motor, which will be non-linear.
In many cases the speed of these servos does not depend too much on the load until you reach a certain threshold. So you can assume that the given speed of 0.17sec / 60 degrees does not change too much under light load but will drastically go down when you approach the maximum torque of 13 kg cm.
If timing is crucial for your application why not test the load/speed relationship yourself?