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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