React Fiber Unplugged: How the Internal Scheduler Shapes React
SessionTrack AEnglish
Underneath every React render is a scheduler making constant decisions: what to work on next, what to interrupt, what to discard, and what to prioritize. Those decisions — not your component code — are usually the reason an app feels fast or janky under load. Yet most developers still reason about React's performance using a mental model from before Fiber existed. This talk goes inside Fiber and its scheduler: how work is broken into units, how priority lanes decide what runs first, how time-slicing lets React yield to the browser mid-render, and why some pre-Fiber optimizations now work against you. We'll trace one real, janky component through the scheduler step by step — what triggers the re-render, how React decides whether to interrupt it, and exactly what changes when you introduce useTransition or useDeferredValue. Everything is source-verified against React's internals, drawn from my 9-part "How React Works" series. You'll leave with a working mental model of the scheduler that explains why certain performance fixes work and others don't — instead of a list of tips applied blindly.
Saman Abaasi
Sam Abaasi is a Staff Frontend Engineer and squad lead at MTN Irancell, Iran's largest mobile network operator, where he architected a Camunda-based workflow platform replacing a legacy production system. He is the creator of an open-source signals-based rendering engine that runs unmodified React components without a virtual DOM (four published npm packages), and the author of three technical deep-dive series including "How React Works" (9 parts, source-verified against React's internals). He is a selected speaker at React Day Berlin 2026 and React Advanced London 2026. A lifelong fan of Japanese culture — from growing up on Sega and Sony to cheering for Japan at the 2026 World Cup — Sam would consider speaking in Tokyo a dream come true.