Ref. PWA02

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

Describe the solution you'd like
As a visitor I want that the time at which the first text or image is painted is less than 2 seconds.

Additional context
https://web.dev/first-contentful-paint
See Metadecidim proposal

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 ...
  • ... First Contentful Paint of homepage is less than 2 seconds.
  • ... First Contentful Paint of a participatory process is less than 2 seconds.
  • ... First Contentful Paint of a proposal is less than 2 seconds.
  • ... First Contentful Paint of a meeting is less than 2 seconds.