  We investigate THz conductivity dynamics in NdNiO$_3$ and EuNiO$_3$ thin
films following a photoinduced thermal quench into the metallic state and
reveal a clear contrast between first- and second-order dynamics. While in
EuNiO$_3$ the conductivity recovers exponentially, in NdNiO$_3$ the recovery is
non-exponential and slower than a simple thermal model. Crucially, it is
consistent with first-order dynamics and well-described by a 2d Avrami model,
with supercooling leading to metastable phase coexistence. The large transients
seen in our films are promising for fast electronic (and magnetic) switching
applications.
