Check a page for Speculation Rules, back/forward cache eligibility and preload hints, the three things that decide whether your next navigation paints instantly. Detects Next.js, Nuxt, SvelteKit, Turbo and other client-side routers first, because they change what is worth doing. LCP is the Core Web Vital most sites fail, and this is the cheapest way to fix it without rewriting the page.
What are Speculation Rules?
A small JSON block in your page that tells Chrome which links to prerender before the visitor clicks. When they do click, the page is already rendered, so it paints immediately. It is a few lines in the head and it is the largest next-navigation win available without touching your content.
What is the bfcache and why does it matter?
The back/forward cache keeps a whole page in memory so pressing back restores it instantly instead of re-rendering. Three common things disqualify a page: a Cache-Control: no-store header, an unload listener, and a beforeunload listener. This tool flags all three from the page source.
Why focus on LCP rather than INP?
Because LCP is where most sites actually fail. INP passes on the large majority of mobile pages; LCP does not. Prerendering and bfcache attack LCP on the navigation the visitor notices most, the second one.
Can this confirm my page really is bfcache-eligible?
It finds the blockers that are visible in the document and headers, which catches the usual causes. A definitive answer needs Chrome's NotRestoredReasons API in a real browser session, so treat a clean result here as very likely rather than proven.
I use Next.js, Nuxt or SvelteKit. Does any of this apply?
Partly, and the tool works it out for you. A client-side router intercepts same-origin link clicks and transitions without loading a document, so a prerendered page for those links is built and then thrown away. When we detect a router we say so, score the rules signal far lower, and take the difference out of the total rather than marking you down for it. Prerendering still pays off for links the router never sees, such as raw anchors inside article bodies, so scope the rules with selector_matches instead of applying them site-wide.
Why does the tool warn about rules that are already installed?
If you have a client-side router and your rules are not scoped by selector, most of that prerendering is wasted work. The warning tells you to narrow the rules to the links that actually cause a document navigation.
Is rel=prerender still supported?
No. The old link rel=prerender was replaced by the Speculation Rules API and browsers no longer act on it. If you still have it, this tool flags it so you can migrate.
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