CPU power is not a problem.
The FFT signal is drawn in another color than the waveform (sorry for the bad photo with wrong colors). My first try was to draw the FFT on top of the waveform (in another color). However, when the number of waves in the waveform is high, the two signals (FFT and wave) can not be seen clearly anymore. Therefore, I changed the code so that the left part of the screen is used for the FFT.