Does the blackening of the forehead from frequent prostration cause a tendency towards boasting, showing off, and loving the praise of others? And should a Muslim prostrate slowly and without prolonging it, or should he prolong his prostration?

The Prophet, peace and blessings be upon him, commanded to perfect the bowing (rukūʿ) and prostration (sujūd). Part of perfecting them is to be at ease and still in them. Failure to perfect them has been deemed to invalidate the prayer. In prostration, it is necessary to firmly place the forehead and nose on the ground. There is no objection to prolonging prostration, especially if one is praying alone, or during the eclipse prayer, or night vigil prayers (qiyām al-layl). The Sunnah (definition) in prostration is moderation and not to spread out the forearms. The mark on the face from the effect of prostration is not an indicator of a person's righteousness; rather, the preponderant opinion is that "their mark is on their faces" refers to the light of obedience and worship.