Why it matters
bfcache makes back/forward navigations instant. An unload listener (or similar blocker) forces a full reload instead.
How to fix it
Replace unload handlers with pagehide or visibilitychange, and only add beforeunload while there are unsaved changes.
Example
addEventListener('pagehide', flushAnalytics);References
More performance rules
void perf reports 25 rules in this category. Core Web Vitals and bytes: LCP, CLS, TBT, TTFB, JavaScript, CSS, fonts, images, caching and compression, measured as the median of several runs on a throttled mobile and a desktop profile.
images-missing-dimensionsImages without width/height or aspect-ratiorender-blockingRender-blocking resources delay first paintno-text-compressionText resources are served uncompressedcache-headersHashed static assets are not cached long-termthird-party-heavyHeavy third-party footprintdom-sizeDOM is very largelighthouse-score-lowLighthouse performance score is lowlighthouse-auditLighthouse flagged an opportunity