I have this code (some kind person has written) but I would like to alter the speed of the servos.
Is this possible?
Whether you can slow a servo, or not, depends on the type of servo being used. This code appears to be written for continuous rotation servos, which CAN be slowed down or sped up.
The usual approach for continuous rotation servos, which really aren't servos, is to use the writeMicroseconds() method to control them. Some minimum value, generally around 1000, results in maximum speed in one direction. As the value increases, the servo turns slower and slower, until a value is reached, generally around 1500, that causes the servo to stop. As the value continues to increase, the servo starts moving again, in the opposite direction. Continuing to increase the value causes the stepper to go faster, until some maximum value is reached, generally around 2000, above which the servo goes no faster.