Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteTo reduce JavaScript’s effect on page load, first find which scripts delay parsing, rendering, or input response. Remove code that is genuinely unnecessary, split features that are not needed on the initial route, and schedule remaining scripts according to their dependencies. Then compare lab diagnostics with field data: smaller downloads can help, but they do not automatically fix a slow page.
Measure where JavaScript is costing time
JavaScript cost is more than the bytes transferred. The browser may need to download, parse, compile, and execute a script; execution can occupy the main thread and delay rendering or response to input. Large assets can also compete with other resources for bandwidth and, on client-rendered pages, delay the browser’s discovery or rendering of main content.
- Inspect requests. In browser developer tools, use the Network panel to identify JavaScript resources, their transfer size, and when they load.
- Inspect execution and coverage. Use the Performance tools and Coverage panel to look for expensive script work and code not exercised during the measured session. Lighthouse audits can also flag unused code and costly JavaScript execution. Google’s unused JavaScript guidance describes these diagnostics.
- Test relevant routes and interactions. Coverage describes only the code observed in that session. Before deleting code, check other routes, user states, and interactions; a script unused in one run may be essential elsewhere.
- Change one cause at a time. Re-run comparable measurements after each change and verify that required behavior still works.
Use lab and field data for different purposes. Lab runs are useful for diagnosing and catching regressions, but a simulated run cannot represent every device, network, route, or interaction. CrUX field data informs tools such as DevTools, PageSpeed Insights, and Search Console. Google recommends real-user monitoring when a site needs detailed per-pageview telemetry to diagnose and respond to regressions; see its Web Vitals guidance and field measurement guidance.
Remove code that is not needed
Prune dependencies, features, and legacy code only after confirming they are not required across the site. Removing genuinely unused JavaScript can reduce transfer, parsing, compilation, memory use, and main-thread work. It may also free bandwidth for other resources.
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- Check whether a dependency is used on any route or only for a feature that can be loaded later.
- Review framework and library imports for unused functionality, and remove dead code where your build process can safely eliminate it.
- Retest routes and interactions, including less common states such as account menus, forms, and consent flows.
Do not equate an “unused” report with proof that a package is safe to delete. A single browser session is a sample, not a complete inventory of site behavior.
Split startup code from features needed later
Keep the initial route’s necessary code in the startup path and load other features when users need them. Dynamic imports and route- or component-level splitting can reduce the amount the browser initially has to parse and compile. For a client-rendered page, less startup work may also help LCP if JavaScript was delaying rendering or discovery of the LCP resource. Google’s code-splitting guidance explains the approach.
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- Use removal when code is not needed anywhere.
- Use lazy loading or route splitting when functionality is real but not needed for the initial view.
- Keep dependencies together when required. A feature split is only useful if it does not cause an avoidable chain of extra requests before the user can use the feature.
Do not optimize for the smallest possible individual files. Many tiny chunks can add network round trips; a large chunk can increase startup work and make cache invalidation less efficient. Smaller files may help repeat visits through caching but can compress less efficiently. Choose chunk boundaries by measuring startup work, compression, caching, and request overhead in production-like conditions.
If a site relies exclusively on client-side rendering, consider whether server-side rendering can return meaningful markup sooner. It is an architectural option, not a substitute for fixing an identified JavaScript bottleneck.
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Choose async and defer based on execution requirements
A classic external <script> without either attribute pauses HTML parsing while it is fetched and executed. The attributes change when scripts execute and whether ordering is preserved; neither is universally correct. See the third-party JavaScript loading guidance.
| Script form | Download and execution behavior | Use when |
|---|---|---|
| Classic script without attributes | Parsing pauses during fetch and execution. | The document genuinely must wait for that script; avoid this for noncritical work. |
async |
Downloads while parsing continues, then executes as soon as available. Execution order is not guaranteed, and execution can interrupt parsing. | The script can run as soon as it arrives and has no ordering dependency on other scripts. |
defer |
Downloads while parsing continues, then executes after parsing completes, preserving document order among deferred scripts. | A noncritical classic script needs document order or the parsed document. |
Check each script’s dependencies before changing its attributes. If script B depends on script A, do not assume two async scripts will execute in the required order. Validate the page’s behavior as well as its load trace.
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Load third-party JavaScript only when it earns its cost
Analytics, advertising, chat, experimentation, and other third-party code can add download and execution work. Keep scripts with clear site value, remove those that do not earn their cost, and consider delaying valuable noncritical scripts until they are needed or until after critical rendering work.
Async loading can reduce parser blocking, but it does not remove a script’s eventual execution cost. A large number of asynchronous scripts can still compete for bandwidth and main-thread time. The web.dev article on third-party script handling reports that the Telegraph deferred scripts including ads and analytics and improved ad loading time by an average of four seconds; that result is specific to that site, not an expected gain for other pages. See the article.
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Check whether users actually improved
Google’s Core Web Vitals are field outcomes, not guaranteed results of a particular JavaScript edit. The thresholds page, last updated May 7, 2025, defines “good” at the 75th percentile as LCP of 2.5 seconds or less, INP of 200 milliseconds or less, and CLS of 0.1 or less. Assess mobile and desktop separately. The same guidance lists “poor” as LCP above 4 seconds, INP above 500 milliseconds, and CLS above 0.25. These thresholds can evolve; consult Google’s current thresholds when evaluating a site.
INP reflects responsiveness across a page experience and requires user interactions. A Lighthouse run with no interaction cannot directly measure field INP. Total Blocking Time (TBT) is a lab proxy that can help locate main-thread blocking during startup, but it is not the same metric as INP. Use field data to determine whether visitors’ responsiveness improved.
Or skip the browser setup
For a screenshot capture, you can use ScreenshotNeo, a website screenshot API and MCP server. It returns a PNG, JPEG, WebP, or PDF from a single GET request. It does not diagnose or optimize your site’s JavaScript; it can help automate page captures for visual checks.
For example, this cURL request saves a WebP capture of the target page. See the ScreenshotNeo documentation for API options.
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