Implement end-to-end (e2e) performance testing

[swissspidy]

Task Description

I wrote a tutorial on end‑to‑end performance testing in WordPress and built https://github.com/swissspidy/wp-performance-testing in the process. The Web Stories plugin could benefit from such performance tests.

There are some frontend performance opportunities worth having tests for so we can measure the impact:

• Web Stories block
  • Improve JavaScript loading using defer/async
    • #13515
  • Use more lightweight JavaScript
    • #13502
  • lazy loading / fetchpriority of story poster images
  • Ensure there are no layout shifts if used in the header or so
• Single stories
  • Potentially add fetchpriority on the preload for the main story image (already preloaded)
    • See https://web.dev/articles/fetch-priority
• Database queries
  • Ensure less stuff is loaded on the frontend
  • #13523
• Speculative resource loading (speculation rules)
  • #13603

Not focusing on the backend right now.

The tests setup would require some simple utilities to create dummy content (stories, blocks, etc.)