Is it possible to decrease the speed of a standard servo motor?
The way I reduce the rotation speed of a standard servo is to just not rotate the servo as fast.

Instead of changing the angle straight from, say, 0° directly to 90°, change the angle in small increments at the speed you want.

Even better is to use a logarithmic or exponential acceleration / deceleration schema.  

Say you want to to from 0° to 90°.  First go from 0° to 1°.  Then from 1° to 3°.  Then from 3° to 6°, then to 10° etc, until you reach a top desired speed - then reduce the distance you are changing with each "step".

So for each "tick" of your control clock you would step by (as an example) 1°, 2°, 3°, 4°, 5°, 5°, ...  5°, 5°, 4°, 3°, 2°, 1°.

Another useful method is the "divide by 2" schema.

For that you simply move by half the target distance with each "tick".  So for a 90° rotation you first move to 45°.  That leaves 45° left to move, so you move 22.5°.  You then have 22.5° left to move, so you move half that at 11.25°, etc, until you have reached your destination.  It makes for a fast start to the movement, with a nice smooth deceleration to the target angle.

These methods are great for reducing the inertia and momentum induced stresses on the load of the servo motor.