Evaluate:

 (add-hook 'after-change-functions (lambda (beg end len) (message "GOT %s->%s [%s]" beg end len)) nil t)

Then yank text.  You'll see the before/after change functions are called 3x upon each yank.  This is due to the final text property changes made by `insert-for-yank-1'.  For substantial amounts of yanked text, this is inefficient.  

Patch attached.