I think the issue is exactly that: 'When a measure becomes a target, it ceases to be a good measure' (https://en.wikipedia.org/wiki/Goodhart%27s_law https://en.

I think the issue is exactly that: 'When a measure becomes a target, it ceases to be a good measure' (https://en.wikipedia.org/wiki/Goodhart%27s_law https://en.wikipedia.org/wiki/Goodhart%27s_law). If you don't specifically optimise for Lighthouse, and then score well in Lighthouse, it's likely a good measure of your site's performance. But if you specifically optimise for Lighthouse and then score well in Lighthouse, it's like practising for a psychiatric examination - it's now measuring your ability to game a test, and not much else.