"Slow 0" is off, aka normal zoom speed. "Slow 99" is as slow as you can go, which is actually still faster than standard speed by an obvious amount in Jennara's case, so I'm guessing it reduces the speed above standard by the percentage indicated.
effect eSlow = EffectMovementSpeedDecrease(iSpeed);
This effect is stopped with immunity to movement speed decreases. This will stack and hinder people more with Slow and other movement speed decreases - the movement speed increases and haste will mearly cancel each other out (thus you walk/run at normal speed with both haste and slow on you) A quick note is that all PC movement rates are 2.00, as defined in CreatureSpeed.2da. That is thier walkrate, thier runrate is 4.00. This is likely tha amount of distance, in meters, they can cover in 1 six-second round. There is no way to directly change a PC's movement speed (say, to 5.00 meters a round). You can make them up to almost immobile, to 99% faster.