Model.insertMany with `lean: true` is very slow compared to native driver

[berkay-dincer]

Prerequisites

• I have written a descriptive issue title

Mongoose version

6.6.1

Node.js version

16.x

MongoDB version

6.0.1

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

No response

Issue

Hey 👋

I am currently working on a piece of functionality that requires bulk inserting of documents using Mongoose. In the process, I've noticed a considerable performance difference between Model.collection.insertMany(docs) method and the Model.insertMany(docs, {lean: true}) method with the {lean: true} flag. Based on the documentation, I understand that the {lean: true} option helps to skip the process of hydrating and validating the documents before insertion. I had thought that this would effectively make these two methods roughly equivalent in terms of performance.

However, upon performing benchmark tests, I found that Model.collection.insertMany is significantly faster than Model.insertMany({lean: true}). I am hoping to better understand why this is the case. I am also attaching a script to replicate the issue.

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const MyModelSchema = new Schema({
    field1: String,
    field2: Number,
    field3: Boolean,
    field4: Date,
    field5: Array
});

const MyModel = mongoose.model('MyModel', MyModelSchema);

async function clearCollection() {
    await MyModel.deleteMany({});
}
async function mongooseInsert(docs) {
    console.time("Mongoose Insert");
    await MyModel.insertMany(docs, { ordered: false, lean: true });
    console.timeEnd("Mongoose Insert");
}

async function nativeInsert(docs) {
    console.time("Native Insert");
    await MyModel.collection.insertMany(docs, { ordered: false });
    console.timeEnd("Native Insert");
}

async function chunkedNativeInsert(docs) {
    console.time("Chunked Native Insert");
    for (let i = 0; i < docs.length; i += 1000) {
        await MyModel.collection.insertMany(docs.slice(i, i + 1000), { ordered: false });
    }
    console.timeEnd("Chunked Native Insert");

}
async function main() {
    await mongoose.connect("mongodb://localhost:27017/test");
    let documents = [];

    for (let i = 0; i < 50000; i++) {
        documents.push({
            field1: `String${i}`,
            field2: i,
            field3: i % 2 === 0,
            field4: new Date(),
            field5: [i, i+1, i+2]
        });
    }
    await mongooseInsert(documents);
    await clearCollection();
    await chunkedNativeInsert(documents);
    await clearCollection();
    await nativeInsert(documents);
    await clearCollection();
}


main();

Thanks!