The maximum speed a servo can move is strictly a function of the servo's design and what mechanical load it is attached to. However in your software you can make the servo move slower by issuing smaller step changes in position commands from start to ending desired position, and by waiting the appropriate amount of time between those issued incremental position commands. It would be pretty simple to write a user general purpose function such as:
void moveServo(int finalPosition, int stepTime);
Lefty