PageSpeed: Minimize Main-Thread Work

Pages with average or slow execution timing on the main thread. The main thread of the renderer process typically handles most code that allows users to interact with a page.

Lighthouse flags pages that keep the main thread busy for longer than 4 seconds during load.

How to Analyse in the SEO Spider

To populate this filter the PageSpeed Insights API must be connected via ‘Config > API Access > PSI’.

View URLs with this issue in the ‘PageSpeed’ tab and filter and use the lower ‘PageSpeed Details’ tab to click on the issue on the left-hand side, and see details of the main-thread work on the right-hand side.

Bulk export the pages and details via ‘Reports > PageSpeed > Minimize Main-Thread Work’.

What Triggers This Issue

This issue is triggered when pages have average or slow execution timing on the main thread, particularly if the main thread is kept busy for longer than 4 seconds during load.

How To Fix

Minimize main thread work by analysing the bottlenecks identified and find ways to defer or remove unnecessary work such as optimising resources.