﴿ أَوْ زِدْ عَلَيْهِ وَرَتِّلِ ٱلْقُرْءَانَ تَرْتِيلًا ﴾

“or add to it [at will]; and [during that time] recite the Qur’an calmly and distinctly, with thy mind attuned to its meaning.”

or add to it up to two thirds aw implies free choice and recite the Qur’ān recite it carefully in a measured tone.