db.collection.createIndex( { String: 1} )
db.collection.createIndex( { Number: 1} )
db.collection.createIndex( { Number: 1, String: -1}  )

{
  "_id" : ObjectId("548f2631ea2de960b50f3fcb"),
  "String" : "Hello world 1587206698",
  "Number" : 1653570026
}

function insert(count) {
    every = 1000
    var t = new Date()
    var randomNum = Math.random();
    var intField = Math.floor(randomNum * 10000000000);
    var stringField_1 = randomNum.toString(36).substring(2, 12);
    var stringField_2 = "Hello World ";
    for (var i = 0; i < count;) {
        var bulk = db.data.initializeUnorderedBulkOp();
        for (var j = 0; j < every; j++, i++)
            bulk.insert({
                ranInt: Math.floor(randomNum * 10000000000),
                ranString: stringField_2.concat(stringField_1)
            })
        bulk.execute();
    }
}
insert(10000000000)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using MongoDB.Bson;
using MongoDB.Driver;

namespace MongoDB
{
    internal sealed class Entity
    {
        public string A { get; set; }
        public int B { get; set; }

    }

    static class Program
    {
        private static bool shouldStop = false;
        private static long counter = 0;

        public static void Main()
        {
            var client = new MongoClient("mongodb://localhost");
            var database = client.GetDatabase("test");
            var collection = database.GetCollection<Entity>("entities");
            var tasks = new List<Task>();
            for (var i = 0; i < 8; i++)
            {
                tasks.Add(Task.Run(() => WriteEntitiesAsync(collection)));    
            }
            tasks.Add(WriteStatisticsAsync());
            Console.ReadLine();
            shouldStop = true;
            Task.WaitAll(tasks.ToArray());
        }

        private static async Task WriteEntitiesAsync(IMongoCollection<Entity> collection)
        {
            var random = new Random();
            var task = Task.Delay(0);
            while (!shouldStop)
            {
                var entities = Enumerable.Range(0, 1000).Select(_ => new Entity()).ToArray();
                foreach (var entity in entities)
                {
                    entity.A = random.Next() + random.Next().ToString() + random.Next();
                    entity.B = random.Next();
                }

                await task;

                var length = entities.Length;
                task = collection.InsertManyAsync(
                    entities,
                    new InsertManyOptions {IsOrdered = false}).
                    ContinueWith(_ => Interlocked.Add(ref counter, length));
            }
        }

        private static async Task WriteStatisticsAsync()
        {
            using (var fileStream = new FileStream("Data.csv", FileMode.Append, FileAccess.Write))
            using (var streamWriter = new StreamWriter(fileStream))
            {
                while (!shouldStop)
                {
                    counter = 0;
                    await Task.Delay(10000);
                    var line = string.Format("{0},{1}", DateTime.UtcNow, Interlocked.Read(ref counter) / 10);
                    await streamWriter.WriteLineAsync(line);
                    streamWriter.Flush();
                    Console.WriteLine(line);
                }
            }
        }
    }
}