Here is an app note from Linear Technology describing the importance of diode turn-on time in high speed (<1MHz) switching regulators. Turn on time is the time it takes the voltage drop on the diode to clamp at the rated voltage drop.
Recently, switching regulator clock rate and transition time have become faster, making diode turn-on time a critical issue. Increased clock rates are mandated to achieve smaller magnetics size; decreased transition times somewhat aid overall efficiency but are principally needed to minimize IC heat rise. At clock speeds beyond about 1MHz, transition time losses are the primary source of die heating.