Bring a large saucepan of water to a boil. Add the shrimp and 1 tablespoon salt and cook until the shrimp are bright pink and cooked through, 2 to 3 minutes. Drain and run under cold water to cool.
In a large bowl, toss the tomatoes, cucumber, bell pepper, parsley, scallions, jalapeño, garlic, oil, lemon juice, cooked shrimp, 3/4 teaspoon salt, and 1/4 teaspoon pepper. Let marinate for 5 minutes before serving.
Shortcut: To get this dish done even faster, use precooked shrimp from the fish counter or the freezer case instead of boiling them yourself. (To thaw frozen shrimp fast, place them in a colander and run under cool water.)