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.