Constant-stimuli threshold measurement is time consuming because many trials are needed per level.

Evidence

Stimuli are then presented to the listener in random order multiple times in order to obtain a reliable percent detection measure. Because this method requires presenting stimuli many times at many different stimulus levels in order to get a measurement of performance at each of those stimulus levels, it can be fairly time consuming. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 288