Enter on a room (for example: https://demo.rocket.chat/channel/general) and run the entire code from the browser console.

test-roles-query-perfomance.js

var uid = Meteor.userId();
var rid = Session.get('openedRoom');

var roles = _.union((ref = UserRoles.findOne(uid)) != null ? ref.roles : void 0, (ref1 = RoomRoles.findOne({
	'u._id': uid,
	rid: rid
})) != null ? ref1.roles : void 0);

var f2 = function() {
	var ref, ref1;

	return RocketChat.models.Roles.find({
		_id: {
			$in: roles
		},
		description: {
			$exists: 1
		}
	}, {
		fields: {
			description: 1
		}
	}).fetch();
};

var f1 = function() {
	var ref, ref1;

	return _.compact(_.map(roles, function(role) {
		var ref2;
		return (ref2 = RocketChat.models.Roles.findOne({
			_id: role,
			description: {
				$exists: 1
			}
		})) != null ? ref2.description : void 0;
	}));
};

console.log('f1 ->',f1());
console.log('f2 ->',f2());

var results = [];

var start = new Date();
for (var i = 0; i < 10000; i++) { f1(); };
var end = new Date();
results.push(end - start);

var start = new Date();
for (var i = 0; i < 10000; i++) { f2(); };
var end = new Date();
results.push(end - start);

console.log(results);