if many users included, don't do N `findOne` operations. Instead, do a single `find` followed by a `forEach`

gistfile1.txt

send(m){
	if (m.numbers){
		m.numbers.forEach((number)=>{
			this._sendSMS(m.text, number);
		})
	} else {
		let sentTo = [];
		Devices.find({userId: {$in: m.users}}).forEach((device) => {
			const devId = this._sendNotification(device, m.title, m.text, m.data);
			devId && sentTo.push(devId)
		})
		Devices.update({_id:{$in: sentTo}},{$set:{lastUsed: new Date()}});
	}
}