  In their early, formative stages star clusters can undergo rapid dynamical
evolution leading to strong gravitational interactions and ejection of
``runaway'' stars at high velocities. While O/B runaway stars have been well
studied, lower-mass runaways are so far very poorly characterised, even though
they are expected to be much more common. We carried out spectroscopic
observations with MAG2-MIKE to follow-up 27 high priority candidate runaways
consistent with having been ejected from the Orion Nebula Cluster (ONC) $>2.5$
Myr ago, based on Gaia astrometry. We derive spectroscopic youth indicators (Li
\& H$\alpha$) and radial velocities, enabling detection of bona fide runaway
stars via signatures of youth and 3D traceback. We successfully confirmed 10 of
the candidates as low-mass Young Stellar Objects (YSOs) on the basis of our
spectroscopic criteria and derived radial velocities (RVs) with which we
performed 3D traceback analysis. Three of these confirmed YSOs have kinematic
ejection ages $>4\:$Myr, with the oldest being 4.7~Myr. This yields an estimate
for the overall formation time of the ONC to be at least $\sim 5\:$Myr, i.e.,
about 10 free-fall times, and with a mean star formation efficiency per
free-fall time of $\bar{\epsilon}_{\rm ff}\lesssim0.05$. These results favor a
scenario of slow, quasi-equilibrium star cluster formation, regulated by
magnetic fields and/or protostellar outflow feedback.
