Why do jet engines take a long time to throttle down?
The simple answer is rotational inertia, once you get a heavy mass spinning fast that heavy mass will have a tendency to continue spinning fast unless something else is acted upon the object. Piston engines have a great deal of friction between the piston walls and pistons (how they hold the explosion in) this helps them slow down faster than a jet in which the turbine is made to easily handle this thus the joints are very well made and very free moving. While cutting fuel will keep the engine from continuing to generate thrust the turbine will continue to spin until a force causes it to stop (drag and friction really).