mongoose: Severe performance regression in 3.8.14

The fix for #2140 (cdd63bf6ff0854018cd6b70fd8397c9e942dcd59) introduced a quite severe performance regression.

Compare the runtime of this code sample with mongoose 3.8.13 and 4.7.2:

var mongoose = require('mongoose')
var fs = require('fs')

var Schema = mongoose.Schema;

ChecklistSchema = new Schema({
    name: { type: String }
});

BoardSchema = new Schema({
    checklists: { type: [ChecklistSchema] }
});

var Board = mongoose.model('Board', BoardSchema);

var doc = new Board();
for (var i = 0; i < 1000; ++i) {
    var checklist = {name:'test'};
    doc.checklists.push(checklist);
}

var obj = doc.toObject();
var time2 = 0;
var start2 = new Date();
for (var i = 0; i < 100; i++) {
  new Board(obj);
}
console.error('instantiating schema object took %d ms', (new Date - start2));

sebastian@host:~/Desktop/test$ node big # mongoose 3.8.13
instantiating schema object took 1127 ms
sebastian@host:~/Desktop/test$ node big # mongoose 4.7.3
instantiating schema object took 6898 ms

So at this point I think it’d be better to either make .id properly handle a null subdoc, or alternatively loop through all subdocs before calling id and checking for null first (and then avoid calling .id if any are found).

Node v6.9.2 mongoose 4.7.3