what temperature is your smoker running at? i'd check to make sure it is in the 230-250 range. if it is higher than 250, you may want to back the heat off. sorry, i just re-read your post and see that your thermometer says 225; you may want to check it for accuracy.[/edit]
like pork shoulders, briskets can go through one heck of a stall; sometimes, they even drop in temperature while the collagen and connective tissues are melting down.
it would be good to know if it were the flat or the point. on a whole brisket, the point (if it were alone) tends to cook faster than a flat (of equal size) would, because the point has much more fat. but the flat is bigger in size (weight) than the point, so when you put the two together, the times even out and both ends are done at the same time. if you bought a point end, it might explain it, but 2.5 hours is still pretty fast.
what i would do is keep an eye on the temperatures, both in your pit and in the thickest part of your brisket (away from the fat) and then bring the brisket off the heat when internal temperatures pass 185 or so. if you have a different "target temperature" in mind, then go with what you planned on.