There will be a temperature difference between the grate and the dome reading. That is normal.
If you are cooking a pork butt at 270 grate temperature you are looking at about 1hr per pound. So depending on the size of the pork butt it very well can reach 170 degrees quick.
P.S. 170 degrees internal temperature, you are at the stall. The collagen and connective tissues are breaking down from solid to liquid. Depending on your grate temperature will also depend on how long it stays at the stall.
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