What make is it and why do want to cook at such low temps? if it is a heavy gauge pit 1 hr is pretty decent to come up to temp. Offsets are all about draft and fire management>It could be you have to much fuel in the box at once blocking up your air flow to the fire. That will greatly reduce the air space needed for good combustion. It is better to have a small hot fire and develop a good coal base and gradually increase the fire size over time until you reach your desired cooking Zone IE 275-325.
I start out with a 1/2 bag of lump and 2 splits to establish a coal base the 1 preheated split every 45-1 hr to maintain my Zone. One thing you can do to increase the draft and get to temp sooner is to place a pan of coals on the grate on the far end of the pit to pull the cool air through the firebox thereby increasing the efficiency, then remove it as you get close to operating temp.
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