  We demonstrate a spectrum demodulation technique for greatly speeding up the
data acquisition rate in scanning nitrogen-vacancy center magnetometry. Our
method relies on a periodic excitation of the electron spin resonance by fast,
wide-band frequency sweeps combined with a phase-locked detection of the
photo-luminescence signal. The method can be extended by a frequency feedback
to realize real-time tracking of the spin resonance. Fast scanning magnetometry
is especially useful for samples where the signal dynamic range is large, of
order millitesla, like for ferro- or ferrimagnets. We demonstrate our method by
mapping stray fields above the model antiferromagnet $\alpha$-Fe$_2$O$_3$
(hematite) at pixel rates of up to 100\,Hz and an image resolution exceeding
one megapixel.
