A few weeks ago, I was building a B2B landing page that embedded an industrial 3D model using <model-viewer>. The page looked fantastic. Then I ran Lighthouse.
Score: 42/100.
Here are the silent killers I found - and what made them non-obvious:
1. The Font Weight Trap (8 requests instead of 3)
Google Fonts was loading wght@300;400;500;600;700;800. That's 6 weights for Inter plus 2 for JetBrains Mono = 8 separate HTTP requests just for fonts.
Each one is render-blocking.
The fix: Reduce to exactly the weights used (400;600;700). Saved ~400ms.
2. <model-viewer> Blocking the Main Thread
The WebGL script was 300KB. Sitting right in the <head>.
The fix: Move it to the very end of the <body>. Use type="module" so the browser defers it automatically.
3. The Silent SEO Killer: JavaScript in JSON-LD
This was the nastiest bug.
A background animation (Particle.js) was running on the page. Due to a weird timing issue, it injected callback code directly into my <script type="application/ld+json"> block.
The page looked perfect visually. No console errors. But the JSON-LD was now invalid. Google Search Console quietly dropped my rich snippets for 3 months before I noticed.
The Open-Source Fix: web-performance-surgeon
I got tired of hunting these issues manually, so I built a Python CLI that scans your HTML and auto-fixes all of them in one go:
# Audit only - shows all issues
python web_performance_surgeon.py index.html
# Audit + auto-fix everything
python web_performance_surgeon.py index.html --fix
Final score after fixes: 100/100 Lighthouse.
I've open-sourced this tool along with my entire B2B web toolchain (including a JSON-LD guardian that specifically catches the injection bug):
GitHub: tanphan1105/web-performance-surgeon
Has anyone else run into that JSON-LD corruption bug? It's terrifying how silently it fails.