Optimize Speed Index

Ref. PWA03

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 contents of a page are visible populated is less than 4 seconds.

Additional context
https://web.dev/speed-index

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 …
• … Speed Index of the homepage is less than 4 seconds.
• … Speed Index of a participatory process is less than 4 seconds.
• … Speed Index of a proposal is less than 4 seconds.
• … Speed Index of a meeting is less than 4 seconds.