Higher RPM road course style downshifts very very quickly completed in less than 1 second (includes the blip and then reengagement of the clutch. However, there must be a pause of about 1 second before you can downshift to each subsequently lower gear. This is not a sequential gearbox, so you the system can't just bang through the gears for you. I've never felt the need to downshift any faster on a road course, than what the GT-R transmission provides me.
On the street, in the sub 2500rpm range, the downshifts are a little more sluggish...this is normal.