Third-party script impact profiled per script

Without profiling, decisions about how to load third-party scripts are guesses (ISO-25010 time-behaviour). A chat widget assumed to be lightweight may consume 120ms of main thread time on load; an analytics script deferred as low-priority may block a critical interaction. Without measured execution times per script, you cannot prioritize optimization work or justify removing a vendor. Unknown third-party impact is one of the most common causes of unexplained performance regressions when a new marketing tag is added.

Profile each third-party script individually using Chrome DevTools and document the findings in your codebase:

1. Open DevTools → Performance → Record a page load.
2. In the flame chart, filter by third-party domain using the Bottom-Up tab → "Group by Domain".
3. Record main thread blocking time per script.

Document results in your project (e.g., docs/third-party-performance.md):

Third-Party Script Audit — 2026-04
- Google Tag Manager: 40ms main thread, load async ✓
- Segment Analytics: 120ms main thread — defer to afterInteractive
- Intercom Widget: 85ms + 200KB — lazy-load on scroll past fold
- HotJar Heatmap: 60ms — acceptable async

Use the WebPageTest "Third Party Summary" view for automated per-script attribution. Revisit this profile quarterly — third-party script weight changes with vendor updates outside your control.

• ID: performance-core.script-style-efficiency.third-party-profile

• Severity: info

• What to look for: Count all relevant instances and enumerate each. Check whether third-party scripts have been profiled for main thread blocking time and bundle size. Look for documentation or comments noting which scripts are slow and their impact. Use DevTools to measure execution time.

• Pass criteria: Each significant third-party script has documented execution time and main thread impact. At least 1 implementation must be verified. Decisions about async/defer/lazy-loading are based on profiling data.

• Fail criteria: No profiling of third-party scripts; decisions are made arbitrarily. Unknown main thread impact from scripts like analytics, ads, or widgets.

• Skip (N/A) when: The project has no third-party scripts or profiling is overkill for the project size.

• Detail on fail: Describe the unknown impact. Example: "Three third-party scripts loaded but no profiling of their main thread impact. Unknown if analytics or ads are biggest bottleneck" or "Chat widget loaded async but impact not measured; could be deferred instead".

• Remediation: Profile third-party scripts using DevTools:

  1. Open Performance tab in DevTools.
  2. Record a page load.
  3. Identify third-party script execution in the flame chart.
  4. Document the main thread blocking time for each script.
  5. Decide on loading strategy based on impact.

  Document findings:

  Third-Party Script Profiling:
  - Google Analytics: 45ms (load async)
  - Segment: 120ms (heavy; load after interactive)
  - Drift Chat Widget: 60ms (load on user hover)
  - Intercom: 85ms (load on user scroll past footer)