Cook the spaghetti according to the package instructions until al dente. Drain well and set aside.
In a small bowl, whisk together the mayonnaise, sweet chili sauce, lime juice, and sriracha until smooth. Set the sauce aside.
In a large skillet, melt the butter over medium heat. Add the shrimp and cook for 2–3 minutes per side, or until pink and opaque. Add the minced garlic and chopped cilantro and cook for 30 seconds, just until fragrant.
Reduce the heat to low and pour the prepared sauce into the skillet, stirring to warm it through.
Add the cooked pasta to the skillet and toss until the noodles are fully coated in the sauce.
Serve immediately, with extra cilantro on top if desired.