There are the results:

3.2.2 - OpenCL

> fft_opencl.exe
ArrayFire v3.2.2 (OpenCL, 64-bit Windows, build 7507b61)
[0] AMD     : Pitcairn
-1- INTEL   : Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz
-2- AMD     : Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz
Benchmark N-by-N 2D fft
 128 x  128:    29 Gflops
 256 x  256:    54 Gflops
 512 x  512:    76 Gflops
1024 x 1024:   115 Gflops
2048 x 2048:   140 Gflops
4096 x 4096:   127 Gflops

3.3 - OpenCL

> fft_opencl.exe
ArrayFire v3.3.0 (OpenCL, 64-bit Windows, build 5842ed2)
[0] AMD     : Pitcairn, 2048 MB
-1- INTEL   : Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz, 8075 MB
-2- AMD     : Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz, 8075 MB
Benchmark N-by-N 2D fft
 128 x  128:    30 Gflops
 256 x  256:    46 Gflops
 512 x  512:    80 Gflops
1024 x 1024:    94 Gflops
2048 x 2048:    49 Gflops
4096 x 4096:    20 Gflops

I'm on Windows 10. On the other hand I can see around a +10% CPU performance increase in 3.3 though, so it gotta be an OpenCL only problem.

Do you aware of this?