MongoDB toArray performance

I'm trying to build a category tree from a term collection in Mongo/Node, but first I select all tree elements using $in:

console.time('termsCol.find');
var terms = await termsCol.find({term_id: {'$in': flatTree}});
console.timeEnd('termsCol.find');

console.time('termsCol.toArray');
terms = await terms.toArray();
console.timeEnd('termsCol.toArray');

This performs:

termsCol.find: 0.162ms
termsCol.toArray: 30.910ms

I've read posts about toArray performance on SO, but would like to know if anything has changed, because this takes most of my time during page request.
I have an index on that collection and it returns 300 terms within ~0.15ms, but that doesn't help me when I have to wait another 30ms to use that data further down in js.
If there's no way to improve this toArray business, I'll probably create a cache collection and store the complete term trees there (hope they fit 16MB).

Solution

reference http://mongodb.github.io/node-mongodb-native/2.0/tutorials/streams/ you can stream results one by one and can make array of ids.

var MongoClient = require('mongodb').MongoClient
var url = 'mongodb://localhost:27017/myproject';
// Use connect method to connect to the Server
MongoClient.connect(url, function(err, db) {
    console.log("Connected correctly to server");
 var col = db.collection('terms');
    var ids = []
    var findCursor = col.find({term_id: {'$in': flatTree}});
    findCursor.on("data", function(data) {
       ids.push(data._id)
    });
    findCursor.on("end", function(data) {
      // let's finish
      console.log(ids)
    }); 
});

i didn't check time but sure it should less then (termsCol.find: 0.162ms + termsCol.toArray: 30.910ms)