Sentence:  `` Speed in painting a picture <WSD>is</WSD> valid only when it imparts spontaneity and crispness , but unless the artist has lots of experience so that he can control rapid execution , he would do well to take these first sketches and soberly reorder their design to achieve a unified composition . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)