You run Lighthouse.
Everything is green.
Scores are high.
Yet real users still complain: “The site feels slow.”
This is one of the most confusing — and common — performance problems in modern web apps, especially in e-commerce. The issue isn’t that Lighthouse is wrong. It’s that Lighthouse doesn’t measure everything users feel.
Let’s break down why this happens and how to fix it.
*Lighthouse Measures Loading, Not Experience
*
Lighthouse is excellent at measuring:
- Initial page load
- Static performance metrics
- Synthetic lab conditions
But users don’t interact with your site in a lab.
They:
- Scroll product grids
- Click filters
- Add items to cart
- Click “Buy”
Most of these interactions happen after Lighthouse has finished measuring.
The Real Culprit: Interaction Latency
A site can load fast and still feel slow if:
- Buttons respond late
- UI freezes after clicks
- Network requests block feedback
- Heavy JS runs during interactions
This delay between user action → visible response is what users actually feel.
Lighthouse barely touches this.
Common Reasons a “Fast” Site Feels Slow
1. JavaScript Blocking the Main Thread
Even small scripts can block interactions if they run at the wrong time.
Symptoms:
- Clicks feel ignored
- UI updates lag
- Scrolling stutters
*2. Slow API Calls After User Actions
*
The page loads fast, but:
- Add-to-cart waits for the backend
- Stock checks hit distant servers
- Cart validation blocks UI updates
Users don’t care why — they just feel delay.
3. Third-Party Scripts on Critical Paths
Analytics, chat widgets, A/B testing tools:
- Often load after Lighthouse finishes
- Execute during user interaction
- Compete for CPU time
One bad script can ruin perceived speed.
*4. CLS and Layout Reflows After Load
*
Even with good CLS scores initially:
- Product grids reflow after filters
- Images resize dynamically
- Fonts swap late
This creates a “janky” feeling.
Lighthouse vs Real User Monitoring (RUM)
Here’s the key difference:
| Lighthouse | Real Users |
|---|---|
| Lab-based | Real devices |
| Fast networks | Slow networks |
| No interaction | Actual clicks |
| One load | Entire session |
If you want to know how fast your site feels, you need RUM data.
A Real-World Observation
On a production e-commerce site, shopperdot.com
, Lighthouse scores were consistently high — yet user sessions showed hesitation after key interactions like filtering products and adding items to the cart.
The issue wasn’t page load.
It was what happened after the page loaded.
By focusing on interaction timing instead of load metrics alone, the perceived performance improved significantly without major architectural changes.
What Actually Improves Perceived Speed
1. Give Instant Visual Feedback
- Button loading states
- Optimistic UI updates
- Skeleton loaders for async actions
Users forgive delays if feedback is instant.
3. Defer Non-Critical JavaScript
- Delay analytics
- Load widgets on interaction
- Split bundles aggressively
Protect the main thread.
4. Measure What Users Feel
Track:
- Interaction delays
- Click-to-response timing
- Long tasks during sessions
If users feel slowness, your metrics should reflect it.
The Big Takeaway
A green Lighthouse score means your site loads fast.
It does not mean your site feels fast.
For e-commerce especially, performance lives in the moments after the load — clicks, taps, and feedback. Optimize those, and user satisfaction improves even if your Lighthouse score stays the same.
💬 Question for you:
Have you ever had users complain about slowness despite perfect Lighthouse scores? What did you discover was the real cause?
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