How to measure the performance of the mode-line?

This interactive function times format-mode-line.

(defmacro mode-line-benchmark-elapse-time (&rest forms)
  "Return the time in seconds elapsed for execution of FORMS."
  (declare (indent 0) (debug t))
  (let ((t1 (make-symbol "t1")))
    `
    (let (,t1)
      (setq ,t1 (current-time))
      ,@forms
      (float-time (time-since ,t1)))))

(defun mode-line-benchmark ()
  (interactive)
  (let* ((value nil)
         (repetitions 10000)
         (wall-clock-time
          (mode-line-benchmark-elapse-time
            (dotimes (_ repetitions)
              (setq value (format-mode-line mode-line-format)))))
         (average-time (/ wall-clock-time repetitions)))
    (message
     "Time: %.10f per call, %.10f for %S calls: %S"
     average-time wall-clock-time repetitions value)))

This can be run manually with M-x mode-line-benchmark