Improve loading on Slow 3G

[surma]

Looking at a WPT trace, we have some opportunities to improve the first load:

• Somehow(?) preload worker dependencies (via prefetch?)
• Delay SW and analytics until after worker is fully initialized
• IUU?

[PaulKinlan]

Some general questions, because I am curious.

• is there a reason that we are not link rel preloading all of the script files (either in HTTP headers or in the head?)
• Should you link rel preload analytics script so it starts earlier? or even just pre-connect?
• The requests 7-11 are all tiny. The connect time seems to be large, is this just because http2 pipe-lining is queuing them up and the connection time is really non-existent? Are we not concatenating them because the expectation that they are likely to be updated in isolation so we are minimising updates when we update parts of the app?
• Is there a reason why our GA code isn't setting it to async in bootstrap.tsx (the standard import code does this).