Memory spike when using `find()`

**Do you want to request a *feature* or report a *bug*?**
Report a bug

**What is the current behavior?**
When I make a `Model#find` call with an `$or` operator, not only does the operation take ~5 minutes, it also takes up ~1 GB memory.

**If the current behavior is a bug, please provide the steps to reproduce.**
My use-case:
```js
const result = await Guild.find({
$or: [
{ premium: { $eq: true } },
{ prefix: { $ne: 'p!' } },
{ blacklistedChannels: { $exists: true, $ne: [], $type: 'array' } },
],
});
```
Reproduction repo: https://github.com/almeidx/mongoose-memory-leak-repro

When I run `node index.js` on this repository:

Not sure if relevant but, here are my collection details:
- documents: 8360
- size: 38.71MB

**What is the expected behavior?**
I expected it to not take ~5 minutes to process, but I can understand why it could take that long to process.
What I really didn't expect is the operation using ~1 GB of memory.

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**
`14.17.1`, `5.12.15` and `3.6.8`, respectively
(I'm not sure if by 'MongoDB' you meant the `mongodb` npm module or the version of the database. I am using MongoDB Atlas version `4.4.6`, `mongodb` npm module version `3.6.8`)

Start with the linked reproduction repository and run node index.js using the reported Node.js 14.17.1, Mongoose 5.12.15, and MongoDB versions. Trace the Model#find call with the shown $or query and compare its runtime and memory behavior; done means identifying whether the behavior belongs to Mongoose or MongoDB and confirming a bounded-memory outcome or documented limitation.

Written by the indexing model from the issue text.