AIM 10-1-2 says helicopter instrument approaches are designed with a maximum speed of 90 KIAS. However, it specifically exempts the final segment on GPS helicopter approaches, saying the maximum speed there is 70 KIAS. Here is the explanation in the AIM:
Obstruction clearance surfaces are based on the aircraft speed and have been designed on these approaches for 70 knots. If the helicopter is flown at higher speeds, it may fly outside of protected airspace.
Why are GPS copter approaches designed with a different obstacle clearance surface that requires a different airspeed on final? Also, if a helicopter flew faster forward with the same descent rate, wouldn't it get farther from the obstacle clearance surface (i.e. making faster speeds safer)?