I think the instructor was exaggerating the consequences of getting slow on the approach. I think what the instructor should have said is what westhawk said - "If approach speed was 5 kts below the scheduled Vapp, the time required for the aircraft to transition from a 3 degree descent angle to a specified climb gradient will increase."
FE Hoppy,
Are you sure the engine would be at a lower RPM when the aircraft is flying a fixed descent angle at a lower speed?
I don't think being in a descent has much to do with it. It has mostly to do with airspeed/AoA. Being at a lower airspeed should produce a slightly lower engine speed. However, there is such a thing as FADEC and I'm not sure how that manages the effect of airspeed on engine speed.