  The Advanced LIGO and Virgo gravitational wave observatories have opened a
new window with which to study the inspiral and mergers of binary compact
objects. These observations are most powerful when coordinated with
multi-messenger observations. This was underlined by the first observation of a
binary neutron star merger GW170817, coincident with a short Gamma-ray burst,
GRB170817A, and the identification of the host galaxy NGC~4993 from the optical
counterpart AT~2017gfo. Finding the fast-fading optical counterpart critically
depends on the rapid production of a sky-map based on LIGO/Virgo data.
Currently, a rapid initial sky map is produced followed by a more accurate,
high-latency, $\gtrsim{12}{\rm{hr}}$ sky map. We study optimization choices of
the Bayesian prior and signal model which can be used alongside other
approaches such as reduced order quadrature. We find these yield up to a $60\%$
reduction in the time required to produce the high-latency localisation for
binary neutron star mergers.
