Letting the Resistance run for 18 hours is basically risk free for the First Order, they have no need to risk anything and the Resistance will die at the end. Its a win-win as far as Hux and Snoke are concerned. No more loss of expensive equipment and personal, and the enemy cannot get away. They just live for another day at the most with their hope fading slowly as they can't get away. Its like the perfect way to end the Resistance for someone like Palpatine. Thrawn probably would have been for a more efficient kill, and avoided the loss of the Dreadnought early in the fight, but he might be all for the running chase if he can abuse the Resistance motives to let them just basically kill themselves without having to waste Imperial assets in combat. By the time it was just the one cruiser left, the First Order could afford to divert all the other Star Destroyers to other parts of the galaxy if they really needed them....the Supremacy would be more than enough to make sure that cruiser can't get away. It just seems that the First Order didn't need those ships anywhere else right then. The Galaxy seems to be just surrendering to them easily after Starkiller Base took out a star system's worth of planets and everyone in the galaxy could see it happen in real time due to the nature of the hyperspace tearing beam weapon thing. So the First Order quite literally has the time to just let the Resistance die a slow death that will take less than a day.
