How Does Reducing Main-Thread Work Improve Core Web Vitals, and What Steps Should I Take to Minimize Main-Thread Blocking Time?


Reducing main-thread work directly improves Core Web Vitals as it enhances Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS), leading to better overall website performance. Several steps can be taken to minimize main-thread blocking time, including optimizing JavaScript tasks, using web workers, and adopting efficient coding practices.

The Impact of Main-Thread Work on Core Web Vitals

The main thread is crucial for rendering web pages, parsing HTML, CSS, and JavaScript, as well as layout and painting tasks [MainThread Work Breakdown, 2021]. High main-thread usage can negatively impact the Core Web Vitals, which are three critical metrics for web performance. These include LCP, measuring page load speed, FID, assessing interactivity, and CLS, evaluating visual stability [Web Vitals, 2021].

Steps to Minimize Main-Thread Blocking Time

Optimize JavaScript Execution

JavaScript plays a principal role in main-thread work. Optimizing JavaScript execution can significantly reduce main-thread workload. This can involve code splitting, removing unused code, and minimizing/uglifying JS [JavaScript Startup Optimization, 2020].

Implement Web Workers

Web workers can be implemented to offload intensive computations to separate background threads, keeping the main thread available for user interactions [Web Workers Overview, 2020].

Adopt Efficient Coding Practices

Efficient coding practices can aid in reducing main-thread blocking time. Practices might include avoiding long tasks, using virtualization for long lists, and limiting layer count [Rendering Best Practices, 2020].

Avoid Large Layout Shifts

Large layout shifts consume significant main-thread work and cause CLS. They can be minimized by specifying sizes for images and embeds, limiting animations, and using transform animations [Cumulative Layout Shift (CLS), 2021].


Reducing main-thread work is key to improving Core Web Vitals, enhancing website performance, and ensuring a smooth user experience. By pursuing different strategies such as optimizing JavaScript, utilizing web workers, adhering to efficient coding practices, and preventing large layout shifts, main-thread blocking time can be significantly minimized.