Measure your Emacs startup time by the help of this command:

time emacs --eval '(kill-emacs)'

and compare with:¹⁾

time emacs -Q --eval '(kill-emacs)'

There are several ways to improve startup time:

1. use Emacs server and emacsclient whenever you can;
2. (auto-) load packages on demand (not in advance, eval-after-load and autoload are your friends);
3. increase the value of custom option gc-cons-threshold (garbage collector threshold);
4. do not output a lot of messages;
5. avoid file I/O if you can:
   • no recentf-mode;
   • turn IDO history off: (setq ido-save-directory-list-file nil);
   • no persistent Org clock (setq org-clock-presist nil);
   • no automatic tags table loading (visit-tags-table).