Problem when apply cilk_for with ippiFFT of intel parallel

[tunheo]

We have a method using ippiFFT to perform forward and inverse FFT of an Image. This method calls ippiFFT 3 times. We try apply cilk_spawn with purpose improve performance of FFT processing, but processing speed of the method is slower than source without cilk_spawn (decreased about 1.5 times). We don't known what is the problem, please help us!

I will describe shortly here:
void performFFT(int start, int end, Ipp32fc[] image)
{
for(int i = start; i < end; i++)
{
ippiFFTInv(Image[i])....
ippiFFTInv(Image[i])....
ippiFFTFwd(Image[i])....
}
}

Source calls method performFFT without cilk_spawn:

Image\[10\];
performFFT(0, 10, Image);

Source calls method performFFT with cilk_spawn: (which is performance decrease 1.5 times)

Image\[10\];
cilk\_spawn performFFT(0,5, Image);
performFFT(5,10,Image);
cilk\_sync;