fft 9.1 ip core

The FFT megacore user guide state that the transform time is 1.08us when you use the 4 multipliers / 2 adders complex multiplier structure, for width 16, ,for 256 poins,and for CycloneIII (EP3C10F256C6) devices.But I have try many metrods and can't achive that 1.08us,dose anybody know the reason?