Speculation Rules with eagerness:"moderate" is the 2026 default for instant navigation; ~82% LCP improvement in CoreDash field data

Claim: Per Arjen Karel's CoreDash field data (corewebvitals.io): "Across sites monitored by CoreDash, prerendered navigations have a p75 LCP of 320ms compared to 1,800ms for standard navigations on the same sites. That is an 82% improvement from a single API."

Confidence: Moderate-to-high — single-source field measurement, but mechanism is well-understood and corroborated by Google Chrome team posts.

Browser support: Chrome/Edge/Opera stable; Safari 26.2 behind a flag; Firefox positive standards position, not shipped. Covers ~70% of global traffic.

Default JSON:

{ "prerender": [{ "where": { "href_matches": "/*" }, "eagerness": "moderate" }] }

moderate prerenders on hover (desktop) or 50ms after a link enters the viewport (mobile, per Chrome 144 from January 2026).

Risk mitigations:

• Use moderate not immediate on most sites.
• Exclude logout/cart-mutation/admin URLs explicitly.
• Use the new (Chrome 144, Jan 2026) prerender-until-script option for pages with analytics that fire on render — fetches HTML, begins rendering, but pauses JS at the first blocking script tag.
• Never use GET requests for state-mutating actions (this is true regardless of Speculation Rules, but they make the lapse exploitable in seconds).