In a large bowl, combine the shrimp, salt, black pepper, garlic powder, onion powder, and cayenne pepper (if using). Toss to coat the shrimp evenly with the spices. Let marinate for 15 minutes.
Sprinkle the cornstarch over the marinated shrimp and toss until well coated.
Heat the vegetable oil in a large skillet over medium-high heat. Once the oil is hot, carefully add the shrimp in a single layer. You may need to do this in batches to avoid overcrowding the pan.
Fry the shrimp for about 2-3 minutes on each side, or until they are golden brown and cooked through. Remove them from the skillet and place them on a paper towel-lined plate to drain excess oil.
Transfer the cooked shrimp to a serving platter, garnish with chopped green onions, and serve with lemon wedges on the side.