Subject: bug#20154: 25.0.50; json-encode-string is too slow for large strings

From: Dmitry Gutov <[email-masked]>
Date: Sat, 21 Mar 2015 00:26:03 +0200

As per your comment, this seems to be the best we can do for long 
strings, without diving into C:

(defun json-encode-string-1 (string)
   "Return a JSON representation of STRING."
   (with-temp-buffer
     (insert string)
     (goto-char (point-min))
     ;; Skip over ASCIIish printable characters.
     (while (re-search-forward "\\([\"\\/\b\f\n\r\t]\\)\\|[^ -~]" nil t)
       (replace-match
        (if (match-beginning 1)
            ;; Special JSON character (\n, \r, etc.).
            (format "\\%c" (char-after (match-beginning 0)))
          ;; Fallback: UCS code point in \uNNNN form.
          (format "\\u%04x" (char-after (match-beginning 0))))
        t t))
     (format "\"%s\"" (buffer-string))))

It brings the execution time down to ~0.14s here, on the same example.

And there'll need to be a fallback for short strings, because 
`with-temp-buffer' overhead is non-trivial.