To save time, cook pasta while making pesto sauce and marinating the shrimp. Boil spaghetti for time indicated on the packaging, usually 8-12 min. During that time put all the pesto ingredients, except oil, into a food processor. Pulse a few times, then drizzle in the oil at high speed until smooth. Set aside. Place the shrimp (dried off) in a bowl and add a dash of salt and lemon juice and mix well. Let sit for 10 min.
When spaghetti has about 5 more min. to cook, heat a large saucepan over medium heat. Add butter, melt. Add shrimp and cook 2-3 min., until opaque and pink. Meanwhile, drain spaghetti.
Once the shrimp are done, turn down the heat to medium-low and pour in the pesto sauce. Mix sauce to incorporate the butter. Then add 3/4 of the drained spaghetti and use tongs to incorporate pesto and noodles. Watch for the sauce to noodle ratio. If it's good as is, don't add any more noodles. If there's extra sauce, add more noodles.