  Ultra-fast vortex motion has recently become a subject of extensive
investigations, triggered by the fundamental question regarding the ultimate
speed limits for magnetic flux quanta and enhancements of single-photon
detectors. In this regard, the current-biased quench of a dynamic flux-flow
regime - flux-flow instability (FFI) - has turned into a widely used method for
the extraction of information about the relaxation of quasiparticles (unpaired
electrons) in the superconductor. However, the large relaxation times
$\tau_\epsilon$ deduced from FFI for many superconductors are often
inconsistent with the fast relaxation processes implied by their single-photon
counting capability. Here, we investigate FFI in $15$ nm-thick $182$
$\mu$m-wide MoSi strips with rough and smooth edges produced by laser etching
and milling by a focused ion beam. For the strip with smooth edges we deduce,
from the current-voltage ($I$-$V$) curve measurements, a factor of 3 larger
critical currents $I_\mathrm{c}$, a factor of 20 higher maximal vortex
velocities of 20 km/s, and a factor of 40 shorter $\tau_\epsilon$. We argue
that for the deduction of the intrinsic $\tau_\epsilon$ of the material from
the $I$-$V$ curves, utmost care should be taken regarding the edge and sample
quality and such a deduction is justified only if the field dependence of
$I_\mathrm{c}$ points to the dominating edge pinning of vortices.
