Al was quiet for awhile, biting his lip. "When I cast, it's like I'm looking up the spell in the dictionary," he said. "Without knowing the alphabetical order.
So in order to find the spell, I have to read through the entire dictionary, or memorise all the pages. But even if I read really, really fast, it's still slow and difficult and it takes going over the dictionary so many times to learn where all the words are."
Al sighed. "Severus just opens his mind on the needed letter, finds the right definition, and casts. He showed me. It took so much less time and thought!"