Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesTo reduce JavaScript’s effect on page load time, first find which scripts delay parsing, consume the main thread, or are not needed for the page’s initial route. Remove code only after checking all routes and interactions; split features that are needed later; and choose defer or async according to execution-order requirements. Then compare repeatable lab diagnostics with real-user field data. A smaller bundle can help, but it will not fix every cause of a slow page.
Measure which JavaScript is slowing the page
JavaScript costs more than the bytes transferred. The browser may also need to parse, compile, and execute each script, and execution can occupy the main thread while the browser is trying to render or respond to input. Large assets can compete with other resources for bandwidth; on client-rendered pages, script work can delay rendering or discovery of the main content.
Use browser diagnostics to find candidates
- In browser developer tools, inspect the Network panel to see which script resources are requested and how much they transfer.
- Use the Coverage panel to identify code not used during the measured page session.
- Run Lighthouse to help locate unused code and expensive JavaScript execution.
Coverage is evidence about one observed route and session, not proof that the code is unnecessary everywhere. Before removing code, check other routes, user states, and interactions that could rely on it. Google’s guidance explains the cost of unused JavaScript and the limits of interpreting a single coverage sample: Reduce JavaScript payloads with code splitting.
Separate lab findings from field experience
Lab runs are useful for reproducing conditions and catching regressions during development, but a simulation cannot represent every device, network, route, or interaction. CrUX field data powers Core Web Vitals information in tools such as DevTools, PageSpeed Insights, and Search Console. For detailed per-pageview diagnosis, Google recommends establishing real-user monitoring. See Web Vitals and field measurement guidance.
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Remove code that the site does not need
Prune dependencies, features, and legacy code only after confirming their role across the site. Removing genuinely unnecessary JavaScript can reduce transfer, parsing, compilation, memory use, and main-thread work. Start with the largest or most expensive candidates, then make one change at a time so you can attribute any improvement or regression.
- Check whether a dependency is used on any route, in any supported state, or after an interaction.
- Replace an oversized dependency with a smaller focused alternative only if it preserves required behavior.
- Keep a record of the removed code and run functional checks for the routes and interactions that may depend on it.
Split startup code from features needed later
Ship only what the initial route needs, then load other code when the user navigates to or invokes the relevant feature. Dynamic imports and route- or component-level splitting can keep later functionality out of the startup payload, reducing the initial parse and compile workload.
Choose between removing and loading later
- Remove it when the feature or dependency is no longer needed anywhere.
- Load it later when it is required, but not for the initial view—for example, a tool opened only after a user action.
- Keep it in startup code when the initial route genuinely needs it to render or function correctly.
Do not split every file just to make chunks small. Too many small chunks can add network round trips; a large chunk can increase startup work and make cache invalidation less favorable. Smaller files may also compress less efficiently, while caching can benefit repeat visits. Measure the balance among startup work, compression, cache behavior, and request overhead in production. The right split depends on the app and its users, not a target number of files.
On a client-rendered page, reducing JavaScript may improve Largest Contentful Paint (LCP) if script work delays rendering or discovery of the LCP resource. If the site relies exclusively on client-side rendering, consider whether server-side rendering could return meaningful markup sooner; code splitting alone does not address every rendering bottleneck. See code splitting guidance.
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Choose script loading behavior by execution needs
A classic external <script> without async or defer blocks HTML parsing while the browser fetches and executes it. The attributes change when execution happens, but they are not interchangeable.
| Script form | Download and execution behavior | When it fits | Main caution |
|---|---|---|---|
| Classic script, no attribute | Parsing pauses while the script is fetched and executed. | Only where that blocking behavior is deliberately required. | Can delay parsing and rendering. |
async |
Downloads while parsing continues; executes as soon as it is available. | Independent scripts whose execution order does not matter and which may run early. | Execution order is not guaranteed, and execution can interrupt parsing. |
defer |
Downloads while parsing continues; executes after parsing completes, preserving document order among deferred scripts. | Scripts that need document order but need not block HTML parsing. | Check dependencies and timing assumptions; it is not suitable for every script. |
For example, a deferred script can be written as <script defer src="/assets/app.js"></script>. Use async only when early execution is appropriate and no ordering dependency is required. Use defer when the script can wait until parsing is complete and document order matters. Confirm each script’s actual requirements rather than applying one attribute to every script. Google’s explanation of loading third-party JavaScript efficiently covers these trade-offs.
Reduce third-party JavaScript deliberately
Every analytics tag, advertising library, embedded widget, and other third-party script should provide clear value to the site. Remove scripts that do not. For those that do, consider whether they can load later or only on the pages that need them. Async loading avoids waiting for download before parsing can continue, but it does not eliminate eventual execution cost; many asynchronous scripts can still compete for bandwidth and main-thread time.
Google’s article reports that Telegraph deferred its scripts, including ads and analytics, and improved ad loading time by an average of four seconds. That is a result reported for that site, not a general expected gain from deferring scripts. The same guidance notes that establishing important third-party origin connections early may save 100–500 ms in context-dependent cases; treat that as a possible optimization to validate, not a guaranteed saving. See the third-party script guidance.
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Validate impact with Core Web Vitals and diagnostics
Google’s threshold guidance, last updated May 7, 2025, defines “good” Core Web Vitals targets at the 75th percentile as LCP at or below 2.5 seconds, Interaction to Next Paint (INP) at or below 200 milliseconds, and Cumulative Layout Shift (CLS) at or below 0.1. Assess mobile and desktop separately. These are outcome thresholds, not a promise that a particular JavaScript change will achieve them. The thresholds and definitions may evolve; see Google’s Web Vitals guidance.
INP reflects responsiveness across the 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 field INP. Use lab metrics to investigate and field data to determine whether visitors’ experience improved. Google describes the distinction in its INP guidance.
- Record a baseline for representative routes in a repeatable lab setup.
- Change one likely cause, such as removing a verified-unused dependency, splitting a later feature, or changing a script’s loading behavior.
- Re-run the same lab checks and verify that required features still work.
- Check field data, segmented by device and page where available, to see whether the change helped actual visitors.
- Keep the change only if the improvement is meaningful and functionality, request overhead, and caching behavior remain acceptable.
Troubleshooting common JavaScript performance problems
A script still blocks parsing after adding async
async does not prevent execution from interrupting parsing. If the script can wait until parsing finishes, test defer; if it is nonessential, consider loading it later. Also confirm you changed the actual script responsible for the delay.
Scripts execute in the wrong order
Async scripts have no guaranteed execution order. If one script depends on another, use a loading strategy that preserves the dependency order, such as deferred scripts in the required document order, or adjust the application’s loading logic.
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Coverage shows a large unused percentage
Coverage only reflects code exercised in the sampled session. Test other routes and interactions before deleting it. If the code is needed later, consider splitting it rather than removing it.
More chunks made the page slower
Splitting can increase request overhead, especially where a route now needs many chunks. Review the request waterfall, startup work, compression, and cache behavior; combine or reorganize chunks based on measured production results.
Lighthouse improves but field experience does not
A lab simulation and field users differ in devices, networks, routes, and behavior. Check field data for the affected pages and device categories, and use real-user monitoring when aggregate field data is not detailed enough to isolate the cause.
Bundle size falls but LCP does not improve
JavaScript may not be the limiting factor for LCP, or the removed code may not have been on the critical path. Check whether the main content is delayed by rendering, resource discovery, or another bottleneck before making more JavaScript changes.
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Or skip the browser setup
For checking how a page appears without setting up a browser capture workflow, ScreenshotNeo is a website screenshot API and MCP server for developers. It does not measure JavaScript performance or replace the diagnostics above. One GET request can return a screenshot or PDF; for a quick screenshot, the cURL example is:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webp
See the ScreenshotNeo API documentation for parameters and formats. Cookie banners are accepted before capture and more than 60 known consent platforms, newsletter popups, and chat widgets are removed; each step can be turned off. Bot checks, blank pages, timeouts, and failed loads are not billed, and neither are cache hits. An MCP server lets AI agents use take_screenshot, get_page_info, and capture_pdf. The free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000.
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