Proficient here means one who reads the Qur'an in accordance with the rules of "Ilm At-Tajwid" (the science of the recitation of the Qur'an) as is evidenced from the words of Imam Bukhari's narration and the title given to the chapter.
This Hadith also highlights the merit of the reader of the Qur'an who does not know 'Ilm At-Tajwid, and for that reason cannot read it fluently. But in spite of this, he reads it with great effort. He will be given double reward, one for the difficulty encountered in reciting it, and another for reading it. Safratun translated in the Hadith as scribes (angels) mean the angels that convey the Wahy (Revelation) to the Messenger of Allah. Imam Bukhari has taken it to mean "one who makes reconciliation". The angels who bring Wahy and chastisement from Allah.