That's what I thought.
So, If I were to code my own servo controller and wanted to add some code to consider load, then there must be something out there to detail what the effect of load would be, no?
Not really. Each model servo will be different for speed:load curves and they are different again when supplied with different PSU voltages (ie higher PSU voltages mean more torque and higher speed).
I've done a number of projects using servos under precise microcontroller control and there's no easy way you can make an arm move exactly the same with a heavy load as it does with a light load.
Regarding what K.I.S.S. said, if you use the controller to generate a slower change in duty cycle the servo will move at roughly the right speed, I have done this. However the position will be significantly behind where it should be if the load is high, and they often break out into instability if the load is high and undamped and you try to move the servo slowly under software control.
There is no way your software can know what the load actually is (with a normal servo). Some of the expensive "digital" servos might have a closed loop reporting system you can monitor under software but that would be an unusual feature and I have not senn nor worked with one of those servos.
One thing that does help a lot is to use a regulated PSU voltage to run the servo, this gives much better performance and repeatability of the servo for speed and load driving ability. The last thing I made had 3 Hitachi analogue servos powered by 3 LM317 adjustable voltage regulators, and the regulators helped to dial in the right speeds and accelerations for the individual servos.
However if yours is for an arm that will lift differing loads, and needs to move the same speed, I would suggest you look at the DC gearmotors with optical encoders (Pololu also have some) as you can control these by software and the optical encoder will give you the actual speed and posiiton.