Trader Joe's uses a single line (in every one I've ever been to) and it is wonderful. Even a line wrapping the entire store will only take 10-15 minutes because they staff appropriately and you're almost always moving.
You also are always next to some kind of food item and the impulse buys are staged in an even longer display than in a regular grocery store. Instead of four feet wide and five feet tall, it may be 20-30 feet wide and 3-4 feet tall.
My Tj's has a single line at the front of the store (likely optimized for those taking public transport and purchasing fewer groceries). But in the back, where the parking lot is, it's a queue per register.
You also are always next to some kind of food item and the impulse buys are staged in an even longer display than in a regular grocery store. Instead of four feet wide and five feet tall, it may be 20-30 feet wide and 3-4 feet tall.