it's like falling off a cliff, where the cliff is L1Data

 % ./fft-benchmark -n 2
Speed test (transform: Forward CFFT)...
time: 0.0 us/transform [total time=0.01952 s / 58817584 instructions, its=1048576]
 % ./fft-benchmark -n 3
Speed test (transform: 0)...
time: 0.0 us/transform [total time=0.02787 s / 83985169 instructions, its=1048576]
 % ./fft-benchmark -n 4
Speed test (transform: 0)...
time: 0.1 us/transform [total time=0.05379 s / 162090970 instructions, its=1048576]
 % ./fft-benchmark -n 5
Speed test (transform: 0)...
time: 0.1 us/transform [total time=0.12044 s / 362941192 instructions, its=1048576]
 % ./fft-benchmark -n 6
Speed test (transform: 0)...
time: 0.2 us/transform [total time=0.24205 s / 729412975 instructions, its=1048576]
 % ./fft-benchmark -n 7
Speed test (transform: 0)...
time: 0.5 us/transform [total time=0.56092 s / 1690296013 instructions, its=1048576]
 % ./fft-benchmark -n 8
Speed test (transform: 0)...
time: 1.2 us/transform [total time=1.23457 s / 3720271429 instructions, its=1048576]
 % ./fft-benchmark -n 9
Speed test (transform: 0)...
time: 2.6 us/transform [total time=2.67798 s / 8069896864 instructions, its=1048576]
 % ./fft-benchmark -n 10
Speed test (transform: 0)...
time: 5.5 us/transform [total time=5.76686 s / 17377997051 instructions, its=1048576]
 % ./fft-benchmark -n 11
Speed test (transform: 0)...
time: 11.8 us/transform [total time=12.40107 s / 37369697733 instructions, its=1048576]
 % ./fft-benchmark -n 12
Speed test (transform: 0)...
time: 25.1 us/transform [total time=26.36260 s / 79441723448 instructions, its=1048576]
 % ./fft-benchmark -n 13
Speed test (transform: 0)...
time: 54.9 us/transform [total time=57.58972 s / 173542297713 instructions, its=1048576]
 % ./fft-benchmark -n 14
Speed test (transform: 0)...
time: 192.9 us/transform [total time=202.22507 s / 609390062282 instructions, its=1048576]
 % ./fft-benchmark -n 15
Speed test (transform: 0)...
time: 475.7 us/transform [total time=498.83994 s / 1503216710994 instructions, its=1048576]