(defvar foo-timer nil)

(defun foo-pre-command ()
  (when foo-timer
    (cancel-timer foo-timer)
    (setq foo-timer nil)))

(defun foo-post-command ()
  (run-with-timer 0.01 nil #'foo-idle))

(defun foo-idle ()
  (sit-for 5))

(add-hook 'pre-command-hook 'foo-pre-command)
(add-hook 'post-command-hook 'foo-post-command)

(flyspell-mode)

;; This causes unread-command-events to sometimes contain
;; e.g. ((t t . 45))

mmmmmm