  The magnetic structure and the multiferroic relaxation dynamics of
NaFeGe$_2$O$_6$ were studied by neutron scattering on single crystals partially
utilizing polarization analysis. In addition to the previously reported
transitions, the incommensurate spiral ordering of Fe$^{3+}$ moments in the
$ac$ plane develops an additional component along the crystallographic $b$
direction below $T\approx 5\text{ K}$, which coincides with a lock-in of the
incommensurate modulation. The quasistatic control of the spin-spiral
handedness, respectively of the vector chirality, by external electric fields
proves the invertibility of multiferroic domains down to the lowest
temperature. Time-resolved measurements of the multiferroic domain inversion in
NaFeGe$_2$O$_6$ reveal a simple temperature and electric-field dependence of
the multiferroic relaxation that is well described by a combined Arrhenius-Merz
relation, as it has been observed for TbMnO$_3$. The maximum speed of domain
wall motion is comparable to the spin wave velocity.
