  The propagation speed of gravitational waves, $c_T$, has been tightly
constrained by the binary neutron star merger GW170817 and its electromagnetic
counterpart, under the assumption of a frequency-independent $c_T$. Drawing
upon arguments from Effective Field Theory and quantum gravity, we discuss the
possibility that modifications of General Relativity allow for transient
deviations of $c_T$ from the speed of light at frequencies well below the band
of current ground-based detectors. We motivate two representative Ans\"atze for
$c_T(f)$, and study their impact upon the gravitational waveforms of massive
black hole binary mergers detectable by the LISA mission. We forecast the
constraints on $c_T(f)$ obtainable from individual systems and a population of
sources, from both inspiral and a full inspiral-merger-ringdown waveform. We
show that LISA will enable us to place stringent independent bounds on
departures from General Relativity in unexplored low-frequency regimes, even in
the absence of an electromagnetic counterpart.
