  The outflow velocity of jets produced by collisionless magnetic reconnection
is shown to be reduced by the ion exhaust temperature in simulations and
observations. We derive a scaling relationship for the outflow velocity based
on the upstream Alfv\'en speed and the parallel ion exhaust temperature, which
is verified in kinetic simulations and observations. The outflow speed
reduction is shown to be due to the firehose instability criterion, and so for
large enough guide fields this effect is suppressed and the outflow speed
reaches the upstream Alfv\'en speed based on the reconnecting component of the
magnetic field.
