It's very situational. If you're talking about writing a static site generator or handcoding a web page, that's mostly true, although if you're trying to not just be efficient, but as efficient as possible, things like optimizing assets are a small but additional step.
If you're maintaining a web app over a period of years, it takes at least some effort and time to keep it slim and efficient because small inefficiencies here and there start to accumulate and these tend to be more demanding even in the best case.
There are some antifeatures that Dan Luu identifies, like dynamically unloading content, that probably take a considerable amount of time to implement while degrading both the user experience and efficiency, but I doubt avoiding those is enough to ensure good performance on more complicated projects.
If you're maintaining a web app over a period of years, it takes at least some effort and time to keep it slim and efficient because small inefficiencies here and there start to accumulate and these tend to be more demanding even in the best case.
There are some antifeatures that Dan Luu identifies, like dynamically unloading content, that probably take a considerable amount of time to implement while degrading both the user experience and efficiency, but I doubt avoiding those is enough to ensure good performance on more complicated projects.