Which snippet should I use?

Start from the symptom you see, not from the snippet name. Each table lists the snippet to run first and what to run next depending on what it shows. The same decision trees drive the agent skills, so an agent follows the same path.

If you are not sure where to start, run LCP, CLS and INP. They tell you which of the sections below applies.

Core Web Vitals are failing

Symptom	Start with	Then
Largest Contentful Paint is slow	LCP	LCP subparts to find the slow phase, then LCP trail to see how the candidate changes
The LCP element is an image	LCP	Image element audit, LCP image entropy and priority hints audit
The LCP element is a video	LCP video candidate	Video element audit
Layout shifts during load	CLS	Layout shifts during loading and interaction, Find above-the-fold lazy-loaded images and Fonts preloaded, loaded and used above the fold
Slow interactions	INP	Interactions, then Input latency breakdown

The page loads slowly

Symptom	Start with	Then
The server responds late	TTFB	TTFB sub-parts to see which phase is slow, TTFB for all resources to find slow requests, Server-Timing and Early Hints for the backend phases, and Service worker analysis if a service worker is involved
Nothing shows for a long time	FCP	Find render-blocking resources, then Critical CSS detection
Scripts block rendering	Script loading	Validate preload, async and defer scripts and JS execution time breakdown
Third-party scripts are suspected	Third-party impact by domain	First and third-party script info and First and third-party script timings
Text files are sent uncompressed	Compression audit	Find render-blocking resources to see whether they also block rendering
Inlined CSS or scripts are heavy	Inline CSS info and size	Inline script info and size
Long pages take a long time to render	Content visibility	DOM size and depth
Fonts load late or shift the layout	Fonts preloaded, loaded and used above the fold	Webfont usage analyzer for faces that load but are never used
Resource hints look wrong	Resource hints validation	Priority hints audit and Prefetch resource validation
The site is built with a JavaScript framework	SSR hydration data analysis	JS execution time breakdown
Repeat visits are not fast	Cache strategy analysis	Service worker analysis

Navigation is not instant

Symptom	Start with	Then
Back and forward reload the page	Back-forward cache	Fix the reasons it lists
The page redirects before it renders	Client-side redirect detection	TTFB
Next pages should feel instant	Speculation rules inspector	Prefetch resource validation

The page feels janky or frozen

Symptom	Start with	Then
Choppy scrolling	Scroll performance	Long animation frames
The page freezes	Long tasks	Long animation frames, then Long animation frames script attribution to name the scripts
Each interaction repeats expensive layout work	Forced synchronous layout	DOM size and depth
Content moves when the user clicks	Layout shifts during loading and interaction	Interactions
Animations stutter	Long animation frames	Long animation frames helpers

Images, video and SVG are heavy

Symptom	Start with	Then
Images are too big or blurry	Oversized images	Image element audit
Above-the-fold images are lazy-loaded	Find above-the-fold lazy-loaded images	Priority hints audit
Offscreen images load eagerly	Find non-lazy-loaded images outside of the viewport	Image element audit
Images combine `loading="lazy"` with `fetchpriority="high"`	Find images with lazy and fetchpriority	Priority hints audit
Videos are slow to start	Video element audit	LCP video candidate
SVG files are large	SVG embedded bitmap analysis	Optimize the file, for example with SVGO

Some users have slow connections

Symptom	Start with	Then
Users on mobile or slow networks struggle	Network bandwidth and connection quality	Oversized images and Compression audit

Running the snippets

Paste any snippet into the DevTools console, or save it as a DevTools snippet, as described in How to use. To run them from a terminal or in CI, use the CLI. To read the result of a snippet as a report, paste its returned object in the visualizer.

Introduction CLS