(Can we date this quote?), Arthur Schopenhauer, chapter 2, in The Art of Literature‎[1]:

A work is objectively tedious when it contains the defect in question; that is to say, when its author has no perfectly clear thought or knowledge to communicate. For if a man has any clear thought or knowledge in him, his aim will be to communicate it, and he will direct his energies to this end; so that the ideas he furnishes are everywhere clearly expressed. The result is that he is neither diffuse, nor unmeaning, nor confused, and consequently not tedious.

(Can we date this quote?), Arthur Schopenhauer, chapter 2, in The Art of Literature‎[2]:

The other kind of tediousness is only relative: a reader may find a work dull because he has no interest in the question treated of in it, and this means that his intellect is restricted. The best work may, therefore, be tedious subjectively, tedious.