I often think of Yeats' lines from Adam's Curse:

I said, ‘A line will take us hours maybe; Yet if it does not seem a moment’s thought, Our stitching and unstitching has been naught.

* I believe there are exceptions, but in general writing should feel like casual conversation. The author can do all kinds of tricks while making it feel like casual conversation, but if they deviate from the feel of spontaneity, they should have good reasons to do so.