A week and a half ago, Bozhidar Batsov stirred up a storm by claiming that Emacs startup time doesn’t matter. There was much foaming at the mouth but I agreed with Batsov: Emacs startup time hardly every matters. When it does, it is in very special circumstances that, happily, most of us never experience.
At the end of my post on the matter, I said that if your Emacs startup time is lengthy, it probably indicates a problem with your configuration. Batsov apparently believes the same and in a new post offers some advice for addressing that problem.
He frames his discussion in terms of the use-package macro that most people use. The secret, he says, is to delay loading packages until they are needed. That makes sense because loading a single package won’t take long but loading, say, a hundred may add significant time to your init time. By deferring the loading, you speed up Emacs’ startup time at the cost of adding a tiny delay later when you first actually need a package.
Batsov’s suggestions are a bit complex but the TL;DR is to avoid the :preface, :init. and :config keywords. You should definitely read his post for the details. Ironically, he doesn’t follow his own advice because, as he says, Emacs startup time doesn’t matter.