The only sensible explanation of the original statement I can come up with is that this instructor meant something different than what was posted here.
During a training session a flight instructor mentioned that if approach speed was 5 kts below the Vapp, engine sppol up time will increase by approx 10 seconds.
Given that engine spool up time is related to the ability of an aircraft to transition from a 3 degree descent angle to to a specified positive climb gradient in the landing configuration, starting this transition from a lower approach speed would result in a greater time required to achieve the specified positive gradient. The actual time required for the engines to accelerate from flight idle to go-around thrust would not be appreciably affected by the airspeed used, while the time required to reach the required positive climb gradient would.
The above quoted statement would more accurately reflect reality if it were changed to read:
" 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."