The Feat improved impaling critical reads as followed (bolded important part):
Improved Impaling Critical:
While you are using Impaling Critical to impale an opponent, and you are still holding onto that weapon, that opponent must succeed at a grapple combat maneuver check against you to pull your weapon out. If you have let go of your weapon, the impaled opponent must spend a standard action to remove the weapon. Until the opponent pulls the weapon out, his speed in all modes is halved and his maneuverability, if any, is reduced by one step. When the weapon comes out, instead of dealing the damage normal for Impaling Critical, you can deal bleed damage equal to your weapon’s damage dice result once per round at the start of that opponent’s turn.
If I impale a person multiple times with different weapons, is their movement halved each time?
40 ft of movement would become 20 after 1 impale, which would become 10 after 2, which would be 5 after 3.
No, typically bonuses and penalties from the same source do not stack unless stated otherwise.