[Issue #6942] Defer and preload assets to improve page loading times

[mtnorthcott]

Closes #6942

PR to defer/preload certain assets to improve page loading times. While this helps the loading times significantly, there is the side effect that some assets will move position on-screen during the loading period (quantified by the cumulative layout shift in the screenshots below)

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[afabiani]

@mtnorthcott very nice thanks

[afabiani]

@mtnorthcott we should revert this because we noticed a lot of regressions while testing the UI.
Deferring stuff improves speed but also introduces several issues:

1. JS errors at loading time/init time
2. UI glitches