Third-Party Script Performance (third-party-script-perf)

Summary

Analyzes external scripts for blocking behavior, payload size, latency, and category (Playwright).

Classification

• Category: PERFORMANCE
• Plugin id: third-party-script-perf
• Version: 1.0.0
• Target types: WEBSITE, WEBSHOP
• Weight class: MEDIUM
• Risk level: not set on plugin object
• Browser required: Yes (Playwright)
• Tier / group: Tier 2 — Browser (Playwright)

Schedule

• scheduleHints: { defaultInterval: '0 */6 * * *', priority: 5 }
• Prerequisites: None declared on this plugin.

Configurable inputs

Public keys are derived from inputSchema: z.object({ ... }) (underscore-prefixed keys are runtime-only and omitted here).

Field	Notes
timeout	Default 30000 ms page load timeout.
slowThresholdMs	Default 1000 ms — scripts slower than this are flagged.
largeThresholdKb	Default 100 KB transfer size per script.

Runtime / injected config

• None documented beyond standard worker context (target, config, logger, reportProgress). Underscore-prefixed keys are internal.

Checks that produce findings

These checks run inside this plugin during a single test run. They are not separate tests in the database — each becomes a Finding linked to the Run for third-party-script-perf.

Check	Typical severity	Why
Render-blocking third-party scripts (renderBlockingStatus === 'blocking')	INFO if ≤3 scripts; WARNING if >3	Parser-blocking third-party JS delays first paint and directly worsens LCP/FCP.
Oversized scripts (transfer size > largeThresholdKb, default 100 KB)	WARNING	Heavy tag-manager/analytics bundles inflate download time on mobile networks.
Slow scripts (resource duration > slowThresholdMs, default 1000 ms)	INFO if ≤5 scripts; WARNING if >5	Long-running third-party loads block the main thread and delay interactivity.
Total third-party script payload > 500 KB	WARNING	Aggregate tag bloat is a common root cause of poor Core Web Vitals on marketing-heavy sites.

How each check works

1. Load page — Playwright networkidle navigation (default 30s), then 2s extra wait.
2. Collect scripts — Reads PerformanceResourceTiming entries where initiatorType === 'script' or URL ends with .js. Excludes same-origin scripts (including www. variants).
3. Categorize — Regex buckets: Tag Manager, Analytics, Ad Pixel, Chat, Heatmap, Consent/CMP, A/B Testing, Social, Video, Font, CDN, Other.
4. Apply thresholds — Blocking count severity splits at >3; slow script severity splits at >5; large scripts always WARNING; total size >512 000 bytes triggers aggregate WARNING.
5. Score and status — Starts at 100, −5 per blocking script, −8 per large script, −3 per slow script, −15 if total >500 KB (floor 0). Status warning if any WARNING finding.

Caveats and concerns

• Same-origin excluded — First-party JS is not analyzed; only cross-domain scripts count.
• Transfer size caveats — transferSize may be 0 for opaque/cached responses, under-reporting weight.
• Blocking API support — renderBlockingStatus is not available in all browser versions; may under-detect blockers.
• Lab snapshot — One load from the worker; ad tags that load conditionally may be missed or over-sampled.
• Triggered by CWV failures — Adaptive orchestrator queues this after failed core-web-vitals runs.

Source

packages/test-plugins/src/plugins/third-party-script-perf.ts