Cook spaghetti in boiling salted water according to package instructions; drain, reserving 1 cup pasta water.
In a skillet, melt 2 tablespoons butter over medium-high heat. Add shrimp, salt, and crushed red pepper; cook until shrimp are pink, about 2-3 minutes, then set aside.
In the same skillet, add remaining butter and garlic, cooking until fragrant, about 1 minute. Remove from heat and stir in lemon juice, pasta water, and parmesan.
Toss the spaghetti in the skillet with the sauce. Add shrimp on top and garnish with parsley. Serve immediately.
Notes
This dish is best served fresh. You can adjust the spice level by varying the amount of crushed red pepper.