MongoDB/RethinkDB Use ODM or not?
The questions you're asking require a lot more background into your environment, stack, and application functionality. Whenever you're parsing the results and generating convenient objects out of them, you're spending CPU time doing that, plus you'll be incurring additional memory overhead generating objects versus just dealing with the raw query results.
Based on my understanding on ODMs, the advantage of using one is the convenience when writing app logic as well as easy code readability. This factor should not be taken lightly for larger projects and can help you organize and implement new features more quickly than without it.
If you're worried about performance, you can always test? But I don't think an ODM is going to make or break your system if you use one.