# Lighthouse caught 5 things my other audit tools had missed — so I built new ones Desktop scored 83 performance, mobile 67, LCP 13.1s on slow 4G. I fixed the site and built 5 new audit tools so the same issues surface in every future audit. Here is each fix and each tool. Author: J.A. Watte Published: April 22, 2026 Source: https://jwatte.com/blog/blog-lighthouse-taught-me-five-new-tools/ --- A Lighthouse audit of jwatte.com turned up eight real issues that every existing audit tool I had also missed. This post shows each issue, the fix I applied, and the new tool I built so the next audit actually flags it. ## The starting scores Desktop: Performance 83, Accessibility 97, Best Practices 92, SEO 100. Mobile: Performance 67, Accessibility 97, Best Practices 92, SEO 100. LCP 13.1s on emulated slow 4G. Everything else on the site was solid. The drag was a handful of small issues I hadn't caught because I didn't have audit tools for them. ## Issue 1: oversized book-cover images Lighthouse flagged 450 KiB of image waste on desktop, 406 KiB on mobile. The W-2 Trap cover is 1600×2560 pixels; it renders at 188×300 on desktop, 328×525 on mobile. I was shipping a 207 KiB image to render at 150 KB of pixels. **Fix:** generated 400w and 800w WebP variants via ImageMagick. Updated `` to pick the right one. Logo went from 15 KB at 1024×1024 to 684 bytes at 64×64 + 1.3 KB at 128×128. **New tool:** [Responsive Image Audit](/tools/responsive-image-srcset-audit/) — loads each ``, reads natural dimensions via `new Image()`, compares against the `width` attribute, and flags anything shipped at ≥2× the displayed width. It also estimates the waste in KB so you know what you get back for fixing it. ## Issue 2: unused preconnect hints `` and `fonts.gstatic.com` were in the ``. The site uses system fonts. Lighthouse: "Unused preconnect." **Fix:** deleted both lines. Kept only the GTM preconnect (actually used). **Existing tool:** [Preconnect + DNS-Prefetch Hygiene Audit](/tools/preconnect-hints-hygiene-audit/) already catches this. Now I use it as a pre-commit check before shipping layout changes. ## Issue 3: GTM blocking the main thread Google Tag Manager shipped 154 KiB, 63 KiB unused, and produced five main-thread tasks over 50ms — one of them 977ms. On mobile this ate LCP. **Fix:** deferred GTM + Clarity loading behind first user interaction or 3-second idle. Analytics still fire (for every real user who interacts); the script no longer blocks LCP. **Existing tool:** [Third-Party Script Cost](/tools/third-party-script-cost/) flagged this already. Separate tool: [Inline Event Handler Audit](/tools/inline-event-handler-audit/) catches the related pattern of inline handlers that block strict CSP. ## Issue 4: Clarity script blocked by CSP Best Practices flagged a CSP violation: `scripts.clarity.ms` was loading but only `www.clarity.ms` was in the allowlist. **Fix:** added `scripts.clarity.ms` + `c.clarity.ms` to the CSP script-src / connect-src directives. **New tool:** [CSP Allowlist Completeness](/tools/csp-allowlist-completeness-audit/) — scans every `