Services

Reduce Third‑Party Script Overhead from Gtm, Analytics and Consent Tools

Use this page for web performance debugging when the site got slower after tag‑manager, analytics, consent, or personalisation changes and too much of the render budget is now going on third‑party tooling.

Reduce third‑party script cost when GTM, analytics, consent, or personalisation tooling starts dragging down key journeys.

Typical symptoms

  • Important pages became slower after consent, analytics, or personalisation changes.
  • Scripting cost is now the dominant issue on commercially important routes.
  • Different third‑party tools are competing for the same critical render budget.

Likely causes

  • Too many third‑party scripts are loading too early or too often.
  • Script ownership is distributed and performance impact is not being managed centrally.
  • Implementation details are not aligned with the actual business priority of each tool.

What I look at first

  • Quick check: rank the scripts that load earliest and do the most work on your highest‑value routes.
  • Whether the main issue is network cost, execution cost, or layout instability.
  • Which tools are business‑critical and which are just legacy weight.

How I help fix this

  • Prioritise third‑party changes against real route and conversion impact.
  • Reduce or defer lower‑value script work without guesswork.
  • Support implementation changes that protect core journeys.

When to bring me in

  • Bring me in when the stack is technically capable but third‑party weight keeps undoing the gains.
  • Bring me in when script ownership is distributed and nobody is prioritising cost against business value.

Related project experience

  1. John Lewis
    & Partners

    Senior developer as part of team 'Findability'. Led the digital implementation of the 'John Lewis & Partners' rebrand alongside new feature development, user journey optimisation, and performance improvements.

    Screenshot of the John Lewis website; part of John Kavanagh's development portfolio.

  2. Selfridges

    Lead front‑end developer within the e‑commerce Customer Experience team. Leading the delivery of new features, as well as considerable refactoring and improvement of the existing platform's performance.

    Screenshot of the Selfridges website; part of John Kavanagh's development portfolio.

  3. Nando’s

    Senior software engineer on the UK and Ireland replatform, migrating Nando’s customer‑facing websites from legacy Drupal to a unified headless platform built with Next.js and Storyblok, with a focus on performance, accessibility, and SEO.

    Screenshot of the Nando’s website; part of John Kavanagh's development portfolio.

Supporting technical articles

Related services

  1. Parent hub

    Next.js Performance and Stability

    Choose the right performance and stability investigation when a live Next.js stack is slower, less stable, or harder to reason about after change.

  3. Adjacent scenario

    Next.js Core Web Vitals Regression Fix

    Recover lost Core Web Vitals after a release before the site feels slower and key routes start hurting conversion, crawl efficiency, or release confidence.

  4. Adjacent scenario

    Next.js Hydration Mismatch Fix

    Diagnose hydration mismatches before Hydration failed errors, brittle UI, and production‑only rendering bugs start compounding.

  5. Related page

    Technical SEO for JavaScript Applications

    Bring in engineering‑led SEO help when Google is not indexing important JavaScript pages because rendering, crawlability, metadata, or migration changes are getting in the way.

Show me what's not working

Send me the affected page or route, point me at the code if that helps, and tell me what you expected to happen versus what is happening now. If this connects to a Next.js migration, technical SEO drop, performance issue, launch, or platform move, include that context too. I'll come back with the clearest next step.

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