Correct Kroma.. Your current speed is factored in.. not the speed indicated on the slider.. as such, if you are doing 21 and cloak, then manually drop speed to 3, the engines have to power down and you gradually lose speed.. Think that the Drones are tracking your power emmissions and not your ship itself.. Cloak is to mask your power, however doing above speed 4 will give greater chance of drones and plasma keeping a lock on you because your engine power has not dropped below the safe zone..
as such an Emergency Decel would put you well below the threshold and allow drones and plasma to lose lock a lot faster...
the faster you go, even cloaked, the better the chance of a drone keeping a lock on you.. also using ECM in combination to cloak will also help break the lock that a drone or Plasma has, but you have to beat the firing ships ECCM to get the benifit of ECM added to cloak.