from standard speed to slower
I second that, see post #6.
(angle) of a RC servo is controlled is controlled by the width of the pulse (modern servos) or the duty cycle of the pulse signal (older servos), see this link
Thus you can instruct the servo to change the position by changing the pulse width of the control signal. When you change the pulse width from one pulse to the next, the servo will start to change position to reflect the newly instructed position. How long it takes the servo to arrive at the new position (speed) has nothing to do with the controlling pulse signal. The servo being a mechanical contraption cannot change the position instantaneously. A motor has to be activated, gears set in motion, finally a load has to be moved. This all takes time. How fast the servo can do this depends on the motor's rpm, the torque, the gear ratio, the load etc. Changing the speed of the servo requires either a faster motor or one with higher torque (or both), another gear ratio, or a lighter load.
Apart from the load these are all properties of the servo, not of the controlling signal. Therefore there is no way to increase
the speed of the servo by any adjustment to the control signal
As mentioned previously also by @Bluejets
, the speed can be lowered
by slowly stepping the control signal's pulse width from the starting position to the ending position. In this case the servo will also step through the positions slowly. In my experience this method is not favourable. Rotating the motor by small increments reduces the mean torque available at the output of the servo and increases jitter of the output.