Is that probe lying on the cooking grate the one that you are using to monitor the pit temp? You should get it off the grate, you may get a false reading with it laying on the cast iron
grill grates. I put mine in a small potato and place it center of the cook chamber.
Another trick to get temps up quickly in a
CharGriller is to keep a few pieces of wood warming on top of the SFB, when temps get low throw in a couple of pieces and give them some air 'til they catch fire, the flames will get temps back up more quickly than just adding unlit charcoal. When I cooked with lump on my CG I had good success anticipating when I would need to add more fuel and usually added lit lump, one chimney full at a time. Another thing I might do if I were in your shoes tonight is put both butts on the same cooking grate, that way they are both sitting in the same temp zone, in your case put the butt with the probe in it next to the darker one it looks like it is cooking faster.
IMHO if your cooking temp stays near 300° it is not a bad thing and you may even get done in under 18 hours.
Good Luck.