Inhibit org-mode-hook in org-agenda

init.el

@@ -571,6 +571,7 @@
   (setq load-path
         (cl-remove-if (lambda (path) (string-match-p "lisp/org\\'" path)) load-path))
   (:also-load +org)
+  (with-eval-after-load '+org (+org-agenda-inhibit-hooks-mode +1))
   (:option org-adapt-indentation nil
            org-auto-align-tags t
            org-archive-mark-done t

lisp/+org.el

@@ -650,5 +650,39 @@ and POST-PROCESS are passed to `org-export-to-file'."
   (set-category-table +org-category-table)
   (setq-local word-wrap-by-category t))
 
+
+;;; Inhibit hooks on `org-agenda'
+;; It's really annoying when I call `org-agenda' and five hundred Ispell
+;; processes are created because I have `flyspell-mode' in the hook.  This mode
+;; inhibits those hooks when entering the agenda, but runs them when opening the
+;; actual buffer.
+
+(defun +org-agenda-inhibit-hooks (fn &rest r)
+  "Advice to inhibit hooks when entering `org-agenda'."
+  (dlet ((org-mode-hook nil))           ; I'm not sure if `dlet' is strictly needed
+    (apply fn r)))
+
+(defvar-local +org-hook-has-run-p nil
+  "Whether `org-mode-hook' has run in the current buffer.")
+
+(defun +org-agenda-switch-run-hooks (&rest _)
+  "Advice to run `org-mode-hook' when entering org-mode.
+This should only fire when switching to a buffer from `org-agenda'."
+  (unless +org-hook-has-run-p
+    (run-hooks 'org-mode-hook)
+    (setq +org-hook-has-run-p t)))
+
+(define-minor-mode +org-agenda-inhibit-hooks-mode
+  "Inhibit `org-mode-hook' when opening `org-agenda'."
+  :lighter ""
+  :global t
+  (if +org-agenda-inhibit-hooks-mode
+      (progn                            ; Enable
+        (advice-add 'org-agenda :around #'+org-agenda-inhibit-hooks)
+        (advice-add 'org-agenda-switch-to :after #'+org-agenda-switch-run-hooks))
+    (progn                              ; Disable
+      (advice-remove 'org-agenda #'+org-agenda-inhibit-hooks)
+      (advice-remove 'org-agenda-switch-to #'+org-agenda-switch-run-hooks))))
+
 (provide '+org)
 ;;; +org.el ends here