**NERD ALERT**
It was "18 parsecs" not "12".
Better get that alert recalibrated, because 12 is correct.
Furthermore, parsec is an astronomical measurement of distance, not time/speed so he could still make it in 18 parsecs.
ack-ack
Yes, but the Kessel Run is through a black hole cluster, which requires a lot of speed to escape the gravity wells. Slower ships have to take a longer and more circuitous route. The Falcon is fast enough to fly closer to the black holes, enabling her to cut distance off the run. So even though the Kessel Run isn't measured by velocity, speed is still the most important consideration.