using DCT for taking spectral derivative

Hi, I have use FFT in matlab to take a function's derivative. Since I always work with real data, can I use DCT to do the same thing (because it is supposed to be faster)?

I found an old thread:

http://www.mathworks.com/matlabcentral/newsreader/view_thread/42166

But there hasn't been any answers yet.

Could anyone help? Thanks!

Bo