I don't think Paul is or was lazy lyrically or musically. It's an accepted fact by the Beatles community that Paul is a perfectionist. Being lazy is contrary to being a perfectionist.
I have always felt that Paul's greatest strength is being able to create new, original musical ideas or concepts, and that his greatest his weakness is not being able to completely develop those musical ideas into a "finished" song.
I think this is why the Lennon/McCartney songwriting partnership worked so well. One's strength was the other's weakness, and vice-versa.
I use the song "Michelle" as an example. It's Paul originated. A beautiful song, original melody, an original theme. Lyrically, the verses are typical McCartney... incomplete, meandering, and pointless. The Lennon-supplied "I Love You, I Love You, I Love You, That's all I want to say." makes the point, and gives the song a strong sense of lyrical completeness. Songs that Paul writes himself almost always lack this element.