JavaScript is responsible for much of what makes modern websites interactive, but it is also one of the primary causes of poor page performance and crawling issues. JavaScript errors, excessive script loading, and unoptimised execution all contribute to slow Core Web Vitals and a worse user experience.
Common JavaScript Performance Issues
Render-blocking scripts prevent the browser from displaying content until the script has downloaded, parsed, and executed. Any script in the head without async or defer attributes is render-blocking. The fix is to add the defer attribute to non-critical scripts and async to scripts that do not depend on DOM order. Scripts that truly need to be synchronous should be minimal and fast-executing.
Unused JavaScript is JavaScript downloaded and parsed by the browser but never actually executed on the page. Common sources include analytics scripts for services no longer in use, A/B testing scripts from retired experiments, jQuery loaded by themes but barely referenced, and WordPress plugins that enqueue JavaScript globally when only needed on specific page types.
Use Chrome DevTools Coverage tab to identify exactly which scripts have high unused percentages. Remove what you can. For the rest, defer loading until the script is actually needed — typically on user interaction rather than on page load.
Long tasks are JavaScript operations that occupy the main thread for more than 50 milliseconds, blocking the browser from responding to user input. These are the primary cause of poor INP (Interaction to Next Paint) scores. Use Chrome DevTools Performance panel to identify long tasks and break them up using setTimeout or the newer scheduler.yield API.
Console Errors and Unhandled Promises
JavaScript console errors and unhandled promise rejections indicate broken functionality. Even errors that do not visibly break the user experience can prevent certain scripts from executing, cause partial page renders, or consume main thread time with error handling.
Check Chrome DevTools Console on every key page type and resolve each error. Pay particular attention to errors that occur during page load rather than only on specific user interactions.
The eval() Problem
eval() executes JavaScript from strings at runtime. It is a security vulnerability because it can execute injected malicious code, a performance issue because it prevents JavaScript engine optimisation, and a red flag in security audits. Replace eval() with safer alternatives: JSON.parse() for parsing JSON, and direct property access or switch statements for dynamic logic.
Script Loading Best Practices
Load third-party scripts only on pages that actually use them. A live chat widget loaded on every page of a 500-page site, when 95% of conversations start from two specific pages, wastes performance budget sitewide. Use conditional loading to restrict scripts to relevant pages.
Consider replacing heavy third-party scripts with lighter alternatives. A full analytics platform loading 80KB of JavaScript on every page can often be replaced with a lightweight alternative that provides the same business insights with 5KB of JavaScript.