chore(perf): Avoid conflicting hints to preload and defer initial JS

[joeyAghion]

This addresses warnings from tools like DebugBear about:

Asynchronous or deferred resource should not be preloaded.
Preloading can negate the benefits of marking resources as "async" or "defer". Consider removing this hint.

Currently, we specify in Link: headers and <link> tags that initial JS bundles should be preloaded. But the actual <script> tags specify they should be deferred. After this PR, the initial JS chunks won't be preloaded, but will be downloaded only once the <script> tags are encountered, and executed whenever the browser decides to execute deferred scripts.

This definitely addresses the DebugBear warnings. The waterfalls seem the same... script execution (~1s in throttled simulations!) happens at the end. It also frees bandwidth to fetch the main image, which is necessary for the LCP event. The thing I'm not 100% sure about is: does this delay later events like dom-ready or TTI? It didn't in my analyses but seems like it could be a possibility.

[damassi]

We're addressing the warnings, but do you remember that wild decrease in stats that we saw from calibre? It was due to adding this.

Are we sure here?

[damassi]

Things have been crazy today and i haven't been following closely, but are you sure that removing defer in collectAssets doesn't address all the things in the best way?

We're loading so much js. We want to hint that.

[joeyAghion]

To be honest, no I'm not sure. My tests were pretty clear that the js should stay deferred ("lab score" went from 70 to 55 otherwise). This change eliminated the warnings, but didn't score any better.

If it were the middle of the week, I'd say let's try it for an official Calibre run or 2. Since it's not, let's wait and revisit next week. I think the more impactful change will be to preload the main image (#14974), ahead of these assets.

[damassi]

We can run a couple manual snapshots if that helps anything, but i see what you mean in terms of getting the weeks average. Either way!

[relativeci]

#1277 Bundle Size — 8.95MiB (0%).

13cf683(current) vs fbc4882 main#1264(baseline)

Warning

Bundle contains 14 duplicate packages – View duplicate packages

Bundle metrics  no changes

	Current #1277	Baseline #1264
Initial JS	3.65MiB	3.65MiB
Initial CSS	0B	0B
Cache Invalidation	39.33%	39.33%
Chunks	103	103
Assets	106	106
Modules	5834	5834
Duplicate Modules	530	530
Duplicate Code	4.03%	4.03%
Packages	266	266
Duplicate Packages	13	13

Bundle size by type  no changes

	Current #1277	Baseline #1264
JS	8.81MiB	8.81MiB
Other	143.36KiB	143.36KiB

Bundle analysis report Branch joeyAghion/hint Project dashboard

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