Mongo range query slow on objects inside array

indexTest.js

var setup = function (numDocs) {
	var batchSize = 1000;

	function makeDoc(maxSubOrders = 10, minDate = ISODate("2014-01-01T00:00:00Z"), maxDate = ISODate("2016-09-27T00:00:00Z")) {

		function randomDate(minDate, maxDate) {
			return new Date(minDate.getTime() + Math.random() * (maxDate.getTime() - minDate.getTime()));
		}

		var numSubOrders = Math.floor(Math.random() * (maxSubOrders + 1));
		var subOrders = [];
		for (var subOrder = 0; subOrder < numSubOrders; subOrder++) {
			subOrders.push({
				suborder_id: subOrder.toString(),
				last_modified: randomDate(minDate, maxDate)
			});
		}

		return {
			last_modified: randomDate(minDate, maxDate),
			suborders: subOrders
		};
	}

	function makeDocBatch(size) {
		var batch = [];
		for (var docCount = 0; docCount < size; docCount++) {
			batch.push(makeDoc());
		}
		return batch;
	}

	function insertDocBatch(batchSize) {
		var batch = makeDocBatch(batchSize);
		var response = db[collectionName].insertMany(
			batch,
			{ ordered: false }
		);
	}

	function index() {
		db[collectionName].createIndex({ "last_modified": 1 });
		db[collectionName].createIndex({ "suborders.last_modified": 1 });
	}

	for (var docs = 0; docs < numDocs; docs += batchSize) {
		insertDocBatch(batchSize);
	}
	if (numDocs % batchSize) {
		insertDocBatch(numDocs % batchSize);
	}
	index();
};

var queryBuilder = (function () {
	function buildQuery(field, startDate, endDate) {
		query = {};
		query[field] = { $lt: endDate, $gt: startDate };
		return query;
	}

	return {
		lastModified: function (startDate, endDate) {
			return buildQuery("last_modified", startDate, endDate);
		},
		subFindLastModified: function (startDate, endDate) {
			return buildQuery("suborders.last_modified", startDate, endDate);
		},
		subElemMatchLastModified: function (startDate, endDate) {
			var query = this.lastModified(startDate, endDate);
			return { "suborders": { $elemMatch: query } };
		}
	}
})();

var numDocs = 1000000;
var collectionName = "subDocs";
var calculateTotalSubOrders = false;
var found = db.getCollectionNames().find(function (name) {
	return name === collectionName;
});

if (!found) {
	setup(numDocs);
}

if (calculateTotalSubOrders) {
	var numOrders = db[collectionName].aggregate([
		{ $project: { numOrders: { $size: "$suborders" } } },
		{ $group: { _id: 1, numOrders: { $sum: "$numOrders" } } }
	]).next().numOrders;
	printjson({
		numDocs: numDocs,
		numOrders: numOrders
	});
}

printjson(db[collectionName].find(queryBuilder.lastModified(ISODate("2016-09-15T00:00:00Z"), ISODate("2016-09-16T00:00:00Z"))).explain("allPlansExecution"));
printjson(db[collectionName].find(queryBuilder.subFindLastModified(ISODate("2016-09-15T00:00:00Z"), ISODate("2016-09-16T00:00:00Z"))).explain("allPlansExecution"));
printjson(db[collectionName].find(queryBuilder.subElemMatchLastModified(ISODate("2016-09-15T00:00:00Z"), ISODate("2016-09-16T00:00:00Z"))).hint({"suborders.last_modified":1}).explain("allPlansExecution"));
//printjson(db[collectionName].find(queryBuilder.lastModified(ISODate("2016-09-15T00:00:00Z"), ISODate("2016-09-16T00:00:00Z"))).limit(10).hint({last_modified:1}).explain("allPlansExecution"));
//printjson(db[collectionName].find(queryBuilder.subLastModified(ISODate("2016-09-15T00:00:00Z"), ISODate("2016-09-16T00:00:00Z"))).limit(10).hint({"suborders.last_modified":1}).explain("allPlansExecution"));

Test for http://stackoverflow.com/questions/39723468/mongo-range-query-slow-on-objects-inside-array
Run via

mongo indexTest.js