Byte-compile it:

  $ emacs -Q -batch -f batch-byte-compile /tmp/timeit.el

Load it with each Emacs build four times:

  $ for i in {0..3}; do ./src/emacs -Q -l /tmp/timeit.elc; done

Calculate mean timings:

  $ for t in /tmp/timings*.txt; do awk '{s+=$1}END{print FILENAME,s/NR}' "$t"; done
  /tmp/timings-27.txt 28.3246
  /tmp/timings-28.txt 31.7968

Emacs 27 build info:

  In GNU Emacs 27.1.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
   of 2020-12-05 built on thunk
  Repository revision: 6916e7954ab5de5e2dc7b25f165a63396d0e70a0
  Repository branch: emacs-27
  Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
  System Description: Debian GNU/Linux bullseye/sid

  Configured using:
   'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native'
   --prefix=/home/blc/.local --program-suffix=27-opt
   --with-x-toolkit=lucid --with-file-notification=yes --with-x
   --with-cairo'

  Configured features:
  XAW3D XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB
  NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT
  LIBOTF ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS
  LIBSYSTEMD JSON PDUMPER LCMS2 GMP

Emacs 28 build info:

  In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
   of 2020-12-05 built on thunk
  Repository revision: dc39c66d3bb6b1db6af0519659ff154bf6d8a5d1
  Repository branch: master
  Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
  System Description: Debian GNU/Linux bullseye/sid

  Configured using:
   'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native' --config-cache
   --prefix=/home/blc/.local --with-x-toolkit=lucid
   --with-file-notification=yes --with-x'

  Configured features:
  XAW3D XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB
  NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT
  LIBOTF ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS
  LIBSYSTEMD JSON PDUMPER LCMS2

Other info:

  value of $LANG: en_IE.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

  $ grep 'model name' /proc/cpuinfo | awk -F: 'NR==1{print $2}'
   Intel(R) Core(TM) i5-3230M CPU @ 2.60GHz
  $ grep -c 'model name' /proc/cpuinfo
  4
  $ gcc --version
  gcc (Debian 10.2.0-19) 10.2.0
  $ uname -a
  Linux thunk 5.9.0-4-amd64 #1 SMP Debian 5.9.11-1 (2020-11-27) x86_64 GNU/Linux

HTH,

-- 
Basil