Bring to a boil over high heat. Continue to boil, turning the pasta frequently with tongs, for 5 minutes. Add 1 pound raw shrimp and cook until the pasta is al dente, the liquid has nearly evaporated, and shrimp are opaque, 3 to 5 minutes more. Remove from the heat, halve the lemon, squeeze the juice over the pasta, and toss to combine. Taste and season with more salt as needed. Top with crushed red pepper flakes, if desired.