If everything is working as designed, it shouldn't be necessary to hold the handle down until the tank empties. The typical human stool is less than 250 grams. The EPA recommends that toilets be designed to flush a minimum of 350 grams. The Drake has been tested to consistently flush 900 grams. Thats 2 pounds of poop.
The original 2 inch flapper has a cup on the underside that holds a bubble of air, causing it to float once opened, until all the water has left the tank. The Toto 3 inch flapper has a carefully calibrated hole in the side of the cup. It floats briefly, but then closes before all the water has left the tank, releasing exactly 1.6 gallons.
A sightly different flush handle operating technique is required. The handle must be pushed all the way down, pulling the flapper fully open, and released immediately. Watch with the lid off, to see if the flapper operates as described. If all is working properly, about half the water will drain from the tank. In the bowl the water rises momentarily, the siphon begins and the water starts to drop, then a wave of water from the large front rim jets sweeps everything into the trapway. Watch fast, it all happens pretty quick.