From Slack:

we've seen cases in the field where customer-created indexes on encrypted QE fields actually actively harm performance - because of pointless writes during insert/update operations and sometimes pathologic impacts on query execution planning

We should display a warning when we detect that:

• A collection uses Queryable Encryption (e.g. by detecting encryptedFields in the config), and:
  • A field that is encrypted is also part of an index, or
  • There is at least one queryable encrypted field, but there is no index that covers the __safeContent__ field.