var StartTime = (Date.now() - (60*60*24*1000)) / 1000;
var EndTime = (Date.now() / 1000);
var Interval = (60*30); // half an hour (this should be dynamic as much as possible)
var SensorType = "acc";

// This query takes 20 seconds (And i have 5 million records in my collection)
SensorData.aggregate([
{
            $match: {
                date: {
                    $gte: StartTime * 1000,
                    $lt: EndTime * 1000
                },
                SensorType: SensorType,
            }
        },

        // Group by Interval
        {
            $group: {
                _id: Object.assign({
                    timestamp: {
                        $subtract: [
                        {
                            $divide: [ '$date', Interval * 1000]
                        },
                        {
                            $mod: [
                            {
                                $divide: ['$date', Interval * 1000]
                            }, 1]
                        }]
                    }
                }),
                value: { $avg: "$Movement" }
            }
        },

        // Sort by date
        {
            $sort: { "_id.timestamp": 1 }
        },

        // Format date & timestamp
        {
            $project: {
                _id: 0,
                timestamp: {
                    $multiply: [Interval * 1000, "$_id.timestamp"]
                },
                value: "$value",
                date: {
                    $add: [new Date(0), {
                        $multiply: [Interval * 1000, "$_id.timestamp"],
                    }],
                }
            }
        }


]);