Wow, that does seem way way too fast.
My last brisket was a 10 pounder, Cooked it at 225, stock #2.
At 4 hours in, IT was 150, at 10 hours in, IT was 165 at 12 hours in it was 178. Took 15.5 hours to reach 195.
Temp swings were from 210 to 250, average 230 degrees, with probe about 1 inch away from the built in probe.
Where do you have your 2nd probe located? If it's too close to the meat, it will make your unit run much hotter. I've found that the area near the meat runs about 30 degrees cooler than the actual box temp due to a cooling envelope that surrounds the meat.