UNDO: Fix lag when doing C-/ (undo/undo-tree-undo)

undo-tree-undo calls undo-list-transfer-to-tree internally which does
a lot of garbage-collect calls to make sure the GC won't run (to counter race
conditions or something)

So, define a hook that sets the GC threshold to maximum, does the
undo (which should go fast now since the GC won't run), then restore
the old GC threshold

config.org

@@ -522,12 +522,32 @@ Some basic C-coding settings (style, indentation offset, ...).
 * Undo-tree
 
 Undo with =C-/=.
+
+The reason for the hook:
+I had a lot of issues with undo lagging (taking literally seconds to complete...).
+undo-tree-undo calls undo-list-transfer-to-tree internally which does
+a lot of garbage-collect calls to make sure the GC won't run (to counter race
+conditions where the GC would corrupt the undo-tree or something).
+
+So, define a hook that sets the GC threshold to maximum,
+does the undo (which should go fast now since the GC won't run)
+and then restore the old GC threshold.
+
 #+BEGIN_SRC emacs-lisp
 (use-package undo-tree
   :ensure t
   :config
     (global-undo-tree-mode)
+    (define-key undo-tree-map (kbd "C-/") 'undo-hook)
 )
+
+(defun undo-hook (&optional arg)
+  (interactive)
+  (setq gc-cons-threshold most-positive-fixnum)
+  (undo-tree-undo arg)
+  (setq gc-cons-threshold 800000)
+)
+
 #+END_SRC
 
 * Volatile highlights