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Emacs on my Mac got upgraded when I upgraded some other package in Homebrew and now working with Markdown files is very slow. I'm now running the railwaycat/emacsmacport/emacs-mac package version with Emacs version 27.1. My markdown-mode is 20201211.329. Disabling markdown-mode makes the problem go away. I have run the profiler (in cpu mode) and gotten the following report:

+ redisplay_internal (C function)                                7782  75%
+ command-execute                                                1850  18%
+ ...                                                             443   4%
+ jit-lock--antiblink-post-command                                162   1%
+ timer-event-handler                                              21   0%
+ #<compiled 0x421b5d07>                                            9   0%
+ gui-set-selection                                                 4   0%
+ clear-transient-map                                               1   0%

Digging into redisplay_internal reveals the following:

- redisplay_internal (C function)                                7782  75%
 - jit-lock-function                                             7687  74%
  - jit-lock-fontify-now                                         7687  74%
   - jit-lock--run-functions                                     7681  74%
    - run-hook-wrapped                                           7681  74%
     - #<compiled 0x1feee74d43f1>                                7681  74%
      - font-lock-fontify-region                                 7671  74%
       - font-lock-default-fontify-region                        7671  74%
        - font-lock-fontify-keywords-region                      7663  74%
         - markdown-match-italic                                 7418  72%
          - markdown-match-italic                                7295  71%
           - markdown-match-italic                               7282  70%
            - markdown-match-italic                              7267  70%
             - markdown-match-italic                             7132  69%
              - markdown-match-italic                            6999  68%
               - markdown-match-italic                           6884  67%
                - markdown-match-italic                          6764  65%
                 - markdown-match-italic                         6656  64%
                  - markdown-match-italic                        6556  63%
                   - markdown-match-italic                       6454  62%
                    - markdown-match-italic                      6446  62%
                     - markdown-inline-code-at-pos-p               5758  56%
                      - markdown-inline-code-at-pos               5757  56%
                       - markdown-match-code                     5753  56%
                        - markdown-search-until-condition               5753  56%
                           apply                                 5752  55%
                         markdown-beginning-of-text-block                  3   0%
                         markdown-end-of-text-block                  1   0%

In short, Emacs appears to be spending looking for the following regular expression in order to identify inline code blocks:

"\\(?:\\`\\|[^\\]\\)\\(?1:\\(?2:`+\\)\\(?3:\\(?:.\\|\n[^\n]\\)*?[^`]\\)\\(?4:\\2\\)\\)\\(?:[^`]\\|\\'\\)"

I know much more than when I posted v1 of this question but I'm still not sure--how do I fix this?

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  • Can we take a peek at what expanding the + sign reveals for the redisplay_internal section? Maybe it's helpful, maybe it's not ....
    – lawlist
    Dec 19, 2020 at 4:51
  • Thanks @lawlist. I was missing expanding the + due to lack of familiarity with the profiler.
    – kuzzooroo
    Dec 19, 2020 at 5:47

1 Answer 1

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It is caused due to common * syntax of lists and italics. For deeply nested lists, parsing gets expensive due to recursive checks for italics. If you are willing to abandon *italics* syntax in favor of _italics_ then the computational overhead can be avoided.

(defconst markdown-regex-italic
    "\\(?:^\\|[^\\]\\)\\(?1:\\(?2:[_]\\)\\(?3:[^ \n\t\\]\\|[^ \n\t]\\(?:.\\|\n[^\n]\\)[^\\ ]\\)\\(?4:\\2\\)\\)")
;; and/or
(defconst markdown-regex-gfm-italic
    "\\(?:^\\|[^\\]\\)\\(?1:\\(?2:[_]\\)\\(?3:[^ \\]\\2\\|[^ ]\\(?:.\\|\n[^\n]\\)\\)\\(?4:\\2\\)\\)"
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