I wrote this command, and it works, but the completions buffer takes a couple seconds to display. Why?

Question

Using the helper command last-message from another StackExchange question, I wrote and bound an anonymous function to switch to the completion buffer if it exists, and if it doesn't exist but would be created by TAB TAB, create that buffer. The function works as intended, except for one problem... It's really slow at displaying the new buffer when it's created. I was first running it in a Python inferior shell for testing, but it has the same behavior in M-x shell.

The function appears to be as fast as you'd expect (basically instant). If I press C-TAB and hit the down arrow, I immediately see the buffer. However, if I press C-TAB and do nothing, it takes 2-3 seconds for the window to pop up.

(defun last-message (&optional num)
  (or num (setq num 1))
  (if (= num 0)
      (current-message)
    (save-excursion
      (set-buffer "*Messages*")
      (save-excursion
        (forward-line (- 1 num))
        (backward-char)
        (let ((end (point)))
          (forward-line 0)
          (buffer-substring-no-properties (point) end))))))

(global-set-key [C-tab]
                (lambda ()
                  (interactive)
                  (switch-to-completions)
                  (if (string= "No completions" (last-message))
                      (progn (completion-help-at-point)
                             (switch-to-completions)))))

Answer 1

Scanning a command's messages is not a good way to determine its result. Maybe this is what you want.

(defun completion-help-at-point-switch ()
  (interactive)
  (completion-help-at-point)
  (when completion-in-region-mode
    (switch-to-completions)))

(global-set-key [C-tab] #'completion-help-at-point-switch)