MongoDB recently announced their new field-level-encryption method for helping tackle DB breaches. (https://www.wired.com/story/field-level-encryption-databases-mongobd/)
In other articles it is covered in some more technical detail (https://www.csoonline.com/article/3403676/new-mongodb-field-level-encryption-can-help-prevent-data-breaches.html) and clearly says "there is no additional performance impact on the server".
Given a simple query in which you'd want to filter by particular values in an encrypted field, how does it perform the same as in the case when decryption (to interrogate a value) is not required? Why is there no performance hit here when you'd have to decrypt fields first before effectively querying them?