commit 58b477e999f3bb5b48c39fe0a4e5ad0d37e2bb9d
Author: Max Nikulin <manikulin@gmail.com>
Date:   Sat Mar 6 22:44:27 2021 +0700

    lisp/org-refile.el: Speed up `org-refile-get-targets'
    
    lisp/org-refile.el (org-refile-get-targets): Optimize performance
    by eliminating backward lookup of already seen headers.
    
    If configuration allows it, incrementally update current outline path.
    For dense target trees (`:maxlevel' and `:level') it allows to avoid
    "one step forward, two steps back" strategy that requires multiple
    backward searches for deeply nested headings.

diff --git a/lisp/org-refile.el b/lisp/org-refile.el
index 4e9f26eff..8e760f1c3 100644
--- a/lisp/org-refile.el
+++ b/lisp/org-refile.el
@@ -267,7 +267,8 @@ converted to a headline before refiling."
   (let ((case-fold-search nil)
 	;; otherwise org confuses "TODO" as a kw and "Todo" as a word
 	(entries (or org-refile-targets '((nil . (:level . 1)))))
-	targets tgs files desc descre)
+	targets tgs files desc descre
+	outline-path cache-outline-path target-outline-level)
     (message "Getting targets...")
     (with-current-buffer (or default-buffer (current-buffer))
       (dolist (entry entries)
@@ -281,6 +282,11 @@ converted to a headline before refiling."
 	 ((and (symbolp files) (boundp files))
 	  (setq files (symbol-value files))))
 	(when (stringp files) (setq files (list files)))
+	(setq cache-outline-path (and org-refile-use-outline-path
+				      (memq (car desc) '(:level :maxlevel))))
+	(setq target-outline-level
+	      (if (and cache-outline-path (eq (car desc) :level))
+		  (if org-odd-levels-only (1- (* 2 (cdr desc))) (cdr desc))))
 	(cond
 	 ((eq (car desc) :tag)
 	  (setq descre (concat "^\\*+[ \t]+.*?:" (regexp-quote (cdr desc)) ":")))
@@ -288,13 +294,13 @@ converted to a headline before refiling."
 	  (setq descre (concat "^\\*+[ \t]+" (regexp-quote (cdr desc)) "[ \t]")))
 	 ((eq (car desc) :regexp)
 	  (setq descre (cdr desc)))
-	 ((eq (car desc) :level)
+	 ((and (not target-outline-level) (eq (car desc) :level))
 	  (setq descre (concat "^\\*\\{" (number-to-string
 					  (if org-odd-levels-only
 					      (1- (* 2 (cdr desc)))
 					    (cdr desc)))
 			       "\\}[ \t]")))
-	 ((eq (car desc) :maxlevel)
+	 ((memq (car desc) '(:level :maxlevel))
 	  (setq descre (concat "^\\*\\{1," (number-to-string
 					    (if org-odd-levels-only
 						(1- (* 2 (cdr desc)))
@@ -318,13 +324,30 @@ converted to a headline before refiling."
 	       (org-with-wide-buffer
 		(goto-char (point-min))
 		(setq org-outline-path-cache nil)
+		(setq outline-path nil)
 		(while (re-search-forward descre nil t)
 		  (beginning-of-line)
 		  (let ((case-fold-search nil))
 		    (looking-at org-complex-heading-regexp))
 		  (let ((begin (point))
-			(heading (match-string-no-properties 4)))
-		    (unless (or (and
+			(heading (match-string-no-properties 4))
+			(heading-level (length (match-string-no-properties 1))))
+		    (when cache-outline-path
+		      (while (and outline-path (<= heading-level (caar outline-path)))
+			(pop outline-path))
+		      (push (cons heading-level
+				  ;; Taken from org--get-outline-path-1. It is really slow.
+				  (if (not heading)
+				      ""
+				    (org-trim
+				     (org-link-display-format
+				      (replace-regexp-in-string
+				       "\\[[0-9]+%\\]\\|\\[[0-9]+/[0-9]+\\]" ""
+				       heading)))))
+			    outline-path))
+		    (unless (or (and target-outline-level
+				    (not (eq heading-level target-outline-level)))
+				(and
 				 org-refile-target-verify-function
 				 (not
 				  (funcall org-refile-target-verify-function)))
@@ -349,7 +372,9 @@ converted to a headline before refiling."
 				   (_ nil))
 				 (mapcar (lambda (s) (replace-regexp-in-string
 						      "/" "\\/" s nil t))
-					 (org-get-outline-path t t)))
+					 (if outline-path
+					     (nreverse (mapcar #'cdr outline-path))
+					   (org-get-outline-path t t))))
 				"/"))))
 			(push (list target f re (org-refile-marker (point)))
 			      tgs)))