Pre-heat the smoker to 225°F, adding water-soaked wood such as apple, cherrywood, hickory or mesquite (note: you may need to add more during smoking).
While you’re waiting for the temperature to come up, prepare the rub and pork loin roast. In a medium bowl, mix the paprika, garlic powder, onion powder, salt, pepper, brown sugar and optional cayenne.
Pat dry with paper towels if needed to remove excess moisture. Trim off excess so there’s no more than ¼-inch layer. Score the fat layer down to the flesh using a sharp knife in a criss-cross pattern.
Rub the meat all over with the mustard or olive oil. Then rub the spices on all sides. Insert a wireless thermometer probe into the middle (if using).
Once the smoker temperature is stable, place the pork loin on the grates with the fat side facing up. Close the lid and adjust the vents if needed.
Smoke until the internal temperature reaches 145-150°F, about 2 ½ to 3 hours for a 3-pound pork loin.
Remove from the smoker and tent with foil. Let rest for 5 minutes so the juices redistribute throughout the meat. Then slice and serve.