What is the ruling on performing supererogatory prayers (nafl) without devotion (khushu'), knowing that a person only gets a reward from their prayer for what they comprehend of it, and that obligatory prayers (fard) performed without devotion are recorded as fulfilled but without reward?

Prayer performed without devotion (khushu') is valid and fulfills the obligation, but the worshipper's reward is diminished to the extent that their devotion is absent. Abandoning supererogatory (nafila) prayers due to a lack of devotion is a deception from Satan. Rather, one must strive to attain devotion and be diligent in performing supererogatory prayers, for they complete the obligatory ones.