  Details of the explosion mechanism of core-collapse supernovae (CCSNe) are
not yet fully understood. There is an increasing number of numerical examples
by ab-initio core-collapse simulations leading to an explosion. Most, if not
all, of the ab-initio core-collapse simulations represent a `slow' explosion in
which the observed explosion energy ($\sim 10^{51}$ ergs) is reached in a
timescale of $\gtrsim1$ second. It is, however, unclear whether such a slow
explosion is consistent with observations. In this work, by performing nuclear
reaction network calculations for a range of the explosion timescale $t_{\rm
grow}$, from the rapid to slow models, we aim at providing nucleosynthetic
diagnostics on the explosion timescale. We employ one-dimensional hydrodynamic
and nucleosynthesis simulations above the proto-neutron star core, by
parameterizing the nature of the explosion mechanism by $t_{\rm grow}$. The
results are then compared to various observational constraints; the masses of
$^{56}$Ni derived for typical CCSNe, the masses of $^{57}$Ni and $^{44}$Ti
observed for SN 1987A, and the abundance patterns observed in extremely
metal-poor stars. We find that these observational constraints are consistent
with the `rapid' explosion ($t_{\rm grow} \lesssim 250$ ms), and especially the
best match is found for a nearly instantaneous explosion ($t_{\rm grow}
\lesssim 50$ ms). Our finding places a strong constraint on the explosion
mechanism; the slow mechanism ($t_{\rm grow} \gtrsim 1000$ ms) would not
satisfy these constraints, and the ab-inito simulations will need to realize a
rapid explosion.
