What is the time complexity of fetching data from a table that is referenced in another table?

const image_schema = () => {
  const common_fields = {
    user_id: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "user",
      required: true,
    file_name: {
      type: String,
      required: true,
  return new mongoose.Schema(common_fields, {
    collection: `image`,
    timestamps: true,

The above is the mongoDB schema for the image collection.

Whenever I need to fetch a subset of rows in this table, I would also need to get the corresponding user info from the user table that is referenced by user_id column.

What is the time complexity of fetching the additional columns from the user table?

Would the speed performance be significantly better if those additional columns from the user collection is included in the image collection, hence breaking normalization?