Preheat oven to 450°F. Line a rimmed baking sheet with foil. In a large bowl, combine 4 cups peeled, sliced peaches (about 4 medium peaches), 1/2 cup sugar, 3 1/2 Tbsp. quick-cooking tapioca and 1 tsp. lemon juice. Let stand for 15 minutes.
In a small bowl, mix 1/2 cup all-purpose flour, 1/4 cup packed brown sugar and 1/2 cup pecan pieces. Using your fingertips or a fork, cut in 4 Tbsp. cold unsalted butter until mixture is crumbly. Sprinkle 1/3 of praline mixture over bottom of 1 (9-inch) unbaked deep-dish pie shell; cover with peach mixture. Sprinkle remaining praline mixture over peaches.
Place pie on prepared baking sheet and bake for 10 minutes. Reduce oven temperature to 350°F and bake until peaches are tender and topping is golden brown, 25 to 30 minutes longer.