Performance of $replaceRoot vs $unwind, in a many to one lookup

Working with Data
,
1

Lets consider the example provided here

For the same of this discussion, let us assume that orders <-> items, is a many to one relationship. ie for every order, there is only one item what would match in a lookup

db.orders.insert([
   { "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
   { "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 }
])

db.items.insert([
  { "_id" : 1, "item" : "almonds", description: "almond clusters", "instock" : 120 },
  { "_id" : 2, "item" : "bread", description: "raisin and nut bread", "instock" : 80 },
  { "_id" : 3, "item" : "pecans", description: "candied pecans", "instock" : 60 }
])

The $replaceRoot aggregation

db.orders.aggregate([
   {
      $lookup: {
         from: "items",
         localField: "item",    // field in the orders collection
         foreignField: "item",  // field in the items collection
         as: "fromItems"
      }
   },
   {
      $replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$fromItems", 0 ] }, "$$ROOT" ] } }
   },
   { $project: { fromItems: 0 } }
])

The $unwind aggregation

db.orders.aggregate([
   {
      $lookup: {
         from: "items",
         localField: "item",    // field in the orders collection
         foreignField: "item",  // field in the items collection
         as: "fromItems"
      }
   },
   {
      $unwind: "$fromItems"
   }
])

I understand that $replaceRoot (with $mergeObject), embeds the fields directly (or merges) into the document, while $unwind creates a sub-document

Q1: Assuming I am fine with either document schemas, I would like to know which one has better performance in terms of RAM used, execution duration.

Q2: How can I measure/view these performance metrics? db.orders.aggregate(…).explain() does not provide the info I need