How to Cut Emacs Startup Time in Half

Is it actually slow with no config? Run emacs -Q to find out!

So why is it so much slower with our configuration?

Add a function to emacs-startup-hook to print out the duration of Emacs startup:

(defun efs/display-startup-time ()
  (message "Emacs loaded in %s with %d garbage collections."
           (format "%.2f seconds"
                   (float-time
                   (time-subtract after-init-time before-init-time)))
           gcs-done))

(add-hook 'emacs-startup-hook #'efs/display-startup-time)

All startup behavior is happening in the normal-top-level function!

A helpful manual page is Summary: Sequence of Actions at Startup.

use-package gives you a few different ways to defer package loading:

• :hook - Package will be loaded the first time one of the hooks is invoked
• :bind - Package will be loaded the first time one of the key bindings is used
• :commands - Package will be loaded when one of the commands are used
• :mode - Package will be loaded the first time a file with a particular extension is opened
• :after - Load this package after other specific packages are loaded
• :defer - If you don’t use any of the other options, this one will defer loading until after startup

There are a few other options use-package provides, but these are all the most likely ones you would use.

The strategy is to look at all of your use-package expressions and decide whether it really needs to be loaded immediately at startup!

If you want to make sure a package gets loaded at startup despite the use of any of the options above, use :demand t.

Let’s try it!

"fde" '(lambda () (interactive) (find-file (expand-file-name "~/.emacs.d/Emacs.org")))

with-eval-after-load is a macro that causes a section of code to be executed only after a particular package gets loaded.

(with-eval-after-load 'org
  (org-babel-do-load-languages
      'org-babel-load-languages
      '((emacs-lisp . t)
      (python . t)))

  (push '("conf-unix" . conf-unix) org-src-lang-modes))

If you need to split up your configuration into multiple sections, this is one way to ensure you don’t accidentally cause a package to load too early!

One other common performance trick is to reduce the number of times the garbage collector will run during the startup process.

Set the gc-cons-threshold high at the beginning of your init.el:

;; The default is 800 kilobytes.  Measured in bytes.
(setq gc-cons-threshold (* 50 1000 1000))

Then bring it back down at the end of your init.el:

;; Make gc pauses faster by decreasing the threshold.
(setq gc-cons-threshold (* 2 1000 1000))

Another option is to use Andrea Corrallo’s gcmh package to manage it automatically.