أَوْ زِدْ عَلَيْهِ وَرَتِّلِ ٱلْقُرْءَانَ تَرْتِيلًا
Aw zid 'alaihi wa rattilil Qur'aana tarteela
or a little more; recite the Quran slowly and distinctly:
or a little more — and recite the Qur'an with measured clarity.
Measured recitation is prescribed here as a manner rather than a quantity, so the surah can later loosen the amount without touching it: 73:20 cuts back the night standing and still asks for recitation. The same word describes how the revelation came down, since 25:32 answers the objection that it did not arrive all at once by saying it was given in stages. Sending and saying are held to one tempo.