"If you’re looking to speed up page load times in your own projects, start by profiling. Then identify the hotspots and defer them until the furthest point of intent — where you know the user is about to perform the action you need to set up."
I would think that it's a bad user experience to experience a big pause right when they want to do something. Sure the page loads faster but a noticeable pause after a click would be tremendously annoying and contribute to the "laggy" feel.
After implementing deferred initialization I asked myself "does reordering feel laggy?" and the answer was no. If it had felt laggy then I'd have started profiling again.
For high-level measurements I use good old "new Date().getTime()". One call before, one call after, subtract the former from the latter and log it to the console.
When I want to dig deeper, I go to the WebKit Inspector's Profile tab. You can turn it on or off through the UI, or use console.profile()/console.profileEnd() in your code to control it programmatically.
Why not use `console.time` and `console.timeEnd` for that? They probably have the same resolution as the underlying date object and they're significantly less of a hassle.