1. Enable profile of slow queries, for example queries that take more than 100ms:
```
    db.setProfilingLevel(1, {"slowms" : 50})
```
2. Execute suspicious operations/queries
3. Find top 10 slowest queries:
```
    db.system.profile.find().limit(10).sort( { ts : -1 } ).pretty()
```
4. Check execution time (millis) and create indexes for the used fields. For example:
```
    db.RuleResult.createIndex( { _p_user: 1 })
```
   It's also possible to create composed indexes:
```
    db.RuleResult.createIndex( { _p_user: 1, debited: 1 });
```