Your page looks loaded but freezes before responding to clicks or taps. Visitors think something is broken and often leave. This is especially noticeable on mobile phones with slower processors.
On WordPress
Perfmatters plugin can delay JavaScript until user interaction (scroll or click). This alone can dramatically reduce TBT. WP Rocket's "Delay JS Execution" feature does the same.
On Shopify
Install the free Shopify Theme Inspector Chrome extension (search "Shopify Theme Inspector" in the Chrome Web Store) — it shows a flame graph of exactly which scripts run on each page and how long each takes, so you know what to target before making changes.
Pro tip: Third-party scripts (chat widgets, review platforms, social sharing buttons) are the #1 cause of high TBT. Try temporarily removing all third-party scripts and measure the difference — you'll immediately see which one is the culprit.
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