Optimize Largest Contentful Paint

Ref. PWA05

NOTE: this feature is from a big development. See PWA discussion at GitHub.

Is your feature request related to a problem? Please describe.
Loading Decidim is pretty slow most of the time.

Describe the solution you’d like
As a visitor, I want that the time at which the largest content element in the viewport is rendered on the screen is less than 5 seconds.

Additional context
https://web.dev/lighthouse-largest-contentful-paint/

Does this issue could impact on users private data?
No, it’s about performance.

Acceptance criteria

• Given that I’m a Decidim developer
  When I go to the Lighthouse Workflow in GitHub Actions
  Then I see that the …
• … Largest Contentful Paint of the homepage is less than 5 seconds.
• … Largest Contentful Paint of a participatory process is less than 2225 seconds.
• … Largest Contentful Paint of a proposal is less than 2.5 seconds.
• … Largest Contentful Paint of a meeting is less than 2.5 seconds.