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JavaScript Isn’t Slow – It’s Just Scheduled Wrong

In this session, we’ll explore why JavaScript’s single-threaded model causes UI lag, why setTimeout() and requestIdleCallback() fail at task prioritization, and how scheduler.postTask() finally gives developers fine-grained control over execution.

Through real-world examples, performance analysis, and a live demo, we’ll show how prioritized scheduling can eliminate UI freezes, improve responsiveness, and make JavaScript execution truly user-centric. If you care about JavaScript performance, this is the talk you don’t want to miss! 🚀

This talk has been presented at JSNation 2025, check out the latest edition of this JavaScript Conference.

FAQ

Srilakna is a JavaScript engineer who specializes in improving JavaScript performance and task scheduling.

The main topic of Srilakna's talk is that JavaScript isn't inherently slow, but rather its scheduling model is inflexible.

JavaScript appears to be slow because of its inflexible scheduling model that doesn't prioritize tasks effectively, leading to potential UI freezes and lag.

JavaScript's scheduling and task prioritization are problematic because the single-threaded, cooperative scheduling model can block important tasks like user interactions if a heavy computation is running.

The scheduler API mentioned by Srilakna is the 'postTask' API, which allows developers to queue tasks with specific priorities to improve performance and responsiveness.

The 'postTask' API allows tasks to be inserted into a global task queue with specific priority levels, such as user-blocking, user-visible, and background, influencing how and when they're executed.

The 'postTask' API has three priority levels: user-blocking (highest priority), user-visible (medium priority), and background (lowest priority).

SetTimeout cannot prioritize or interrupt tasks, and requestIdleCallback only runs tasks when the thread is idle, which may never happen if the user continuously interacts with the browser.

The 'postTask' API is currently available in Chromium-based browsers, with polyfills available for others. Safari and Firefox are also tracking its implementation.

The benefit of using the 'postTask' API is that it provides explicit control over task priority, ensuring important tasks run promptly and preventing UI freezes and lags.

performance

Sulagna Ghosh

14 min

16 Jun, 2025

Comments

Shubhankar Mitra
Awesome!
19:07 Jun 16, 2025

Video Summary and Transcription

Srilakna discusses JavaScript performance, highlighting scheduling challenges and the limitations of existing models. The introduction of the Post Task Scheduler API addresses these issues, offering promise-based scheduling for specific priorities. The internal workings and implementation of the API are explained, emphasizing task prioritization to ensure smooth UI performance. The API enables developers to control task priorities effectively, preventing UI freeze and lag.

Available in Español: JavaScript No Es Lento – Solo Está Programado Incorrectamente

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