UNDERSTANDING WHY REACT FIBER EXISTS

Summary from the TLDR Dev newsletter

React 15 used recursion to walk the component tree, which filled the JS call stack and couldn't be interrupted. This meant any heavy render would block user input until it finished. Fiber replaces that recursive model with a linked-list data structure React owns, letting it process work in 5ms chunks and yield control back to the browser between each slice, making React much more responsive.