I've just built the latest fftw3 with gcc-4.6.0 (it uses the compiler options -O3 -fast-math) and compared it to gambit-4.6.0 (without any updates) configured with ./configure CC='/pkgs/gcc-4.6.0/bin/gcc -fschedule-insns' For both systems we computed an fft of $2^{25}$ complex elements. The scheme code did an "out-of-place" fft, fftw3 does an "in-place" fft, but I can't see any way to get fftw3 to do an "out-of-place" fft, even though there are reports of such a code on the BenchFFT web site. The results are Gambit: 2730 ms cpu time (2730 user, 0 system) FFTW3: With FFTW_ESTIMATE (estimate the best algorithm without any actual testing): 4.5400000000000000e+00 seconds With FFTW_PATIENT (compute the specific size FFT we want to do with many different algorithms, then choose the fastest one): 2.9500000000000002e+00 seconds Brad PS: The total time for the code that tests many different algorithms is 1587.630u 4.840s 26:34.24 99.8% 0+0k 0+0io 0pf+0w