2D FFT for Huge Matrix

I have 2d Vector with 2533136*8 dimension like below :

vector<vector<double>> DataChannel1(2533136, vector<double>(8));

I want to take FFT from this Vector with fastest solution but I don not How!

can you help me How can I do that?

In addition to I used FFTW library but with my dimension(ROW=2533136 and COL=8) Take long time to calculate.

const auto ROWS = 253313;
const auto COLS = 8;
fftw_complex in[ROWS][COLS], out[ROWS][COLS];
fftw_plan g;


g = fftw_plan_dft_2d(ROWS, COLS, *in, *out, FFTW_FORWARD, FFTW_MEASURE);