The description of MKL perforce states:
"Fast Fourier Transforms
Intel MKL FFTs include many optimizations and should provide significant performance gains over other libraries ****** for medium and large transform sizes. ******" (emphasis mine).
What is the expected value of "medium"? I am comparing performance of MKL FFT against FFTPACK 5.1 and am seeing a break-even value that *I* think is quite large; way bigger than I expected.