Claude Code PSI: 88 to 100 Mobile Performance

A developer used Claude Code as a debugging and implementation partner to push their site's PageSpeed Insights mobile score from 88 to 100 (an article template went from 74 to 100). The post on r/ClaudeAI details the hands-on workflow — not a one-shot fix, but a series of targeted interventions.

Key Optimisations Achieved

Responsive images + srcset: Claude identified that the mobile hero image was larger than needed, then generated responsive image variants and added srcset attributes.
LCP preload: Added a responsive preload for the LCP image using <link rel="preload" as="image" imagesrcset="...">.
Forced reflow fix: Replaced a header scroll listener (which caused forced layout on scroll) with IntersectionObserver.
Font audit: Identified which font weights were actually in use and removed unused ones.
Build-time image dimensions: Added width and height attributes to markdown images at build time to eliminate layout shifts.
Removed font preloads on article pages: Counterintuitive but effective — preloaded fonts were competing with the hero image for bandwidth on throttled mobile. Since fonts used font-display: swap, removing preloads let the LCP image load faster.

Effective Prompting Pattern

The developer found Claude Code works best with specific diagnostic threads. Instead of vague prompts, feed it PSI findings and ask for the smallest safe patch. Example pattern: feed the PageSpeed Insights finding, ask Claude to inspect the relevant component, then request a minimal fix rather than a broad refactor.

Human Judgment Still Required

Some optimisations felt too aggressive visually — the author bumped logo quality back up while staying at 100. Below-the-fold images and styling trade-offs needed human decisions. Claude Code accelerated the iteration, but the developer still had to balance performance, accessibility, and design quality.