To add an advertising network, social media button, A/B test, or analytics service to your page, you usually need to add a third-party script to your HTML. These third-party scripts can significantly affect your page load performance.
How the Lighthouse audit for third-party code fails
Lighthouse flags pages that have third-party code that blocks the main thread for 250 ms or longer:
A third-party script is any script hosted on a domain that's different to the domain of the URL that you audited with Lighthouse. As the page loads, Lighthouse calculates how long each of the third-party scripts blocks the main thread. If the total blocking time is greater than 250 ms the audit fails.
How to reduce the impact of third-party code