I have a code which pretty prints data in json:

...

  (with-temp-buffer

    (insert data)

    (json-pretty-print-buffer)

    (write-file "data.json"))

...

This code runs for several minutes (I didn't time it, but it's about 1-2 minutes) with Emacs 27.1  It takes several seconds with Emacs 26.  Data is about 1MB on disk in pretty printed elisp format.

I tried profiling with this result:

          - progn                                                7486  98%

           - json-pretty-print-buffer                            7438  98%

            - json-pretty-print                                  7438  98%

             - replace-region-contents                           7438  98%

              + #<compiled 0x3854909>                             198   2%

           + insert                                                43   0%

           + write-file                                             5   0%

Apparently, replace-region-contents  takes 98% of the CPU time.

Using Windows version from GNU ftp :

GNU Emacs 27.1 (build 1, x86_64-w64-mingw32) of 2020-08-12