That said, today, where I work we sit behind a fairly hefty cache and CDN, the attitude is just cache what we can and returning visitors will have fast page loads, that's wrong and presumptuous in my opinion.
Where I work our approach is similar, except we also put everything on the edge that we can, and use some proprietary stuff we built to keep us in sync
Hefty caching and CDN are great tools and mitigate the problems but the problem of CPU time running all that code doesn't go away.
I'd say that the perception of speed is far more important than actual fast code, cached or not. What your talking about is user experience after the fact. Keep calm and lazy load, defer and look at script attributes. Take charge and don't trust browsers. Until multiplexing http2 arrives that is.
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