There's always some element of subjectivity when evaluating slow play, but I think that most of the time writing down the whole decklist would be too slow

The rules say that you have to play at a reasonable pace, so the way I would look at it is that if you're spending much longer than you normally would to search a library because you're writing down every card, you're not taking the game action of searching the library at a reasonable pace.

But you can look at the whole thing, and probably write down some important or unexpected cards

Also, it's probably worth mentioning that technically speaking, you're not allowed to reorder a library while searching it. So you wouldn't be able to sort the library before trying to write it down