React 15 was defined by its Stack Reconciler, a synchronous, uninterruptible update process. When a state change occurred, React locked the main thread, recursively calling render() methods and blocking browser input until the entire component tree finished. This high-speed train with no brakes approach caused significant performance issues in complex applications.

The core problem was simultaneous diffing and patching. React's algorithm calculated the Virtual DOM and immediately mutated the Real DOM in one unbroken chain. This made pausing impossible and left the browser hostage to JavaScript's single-threaded nature, causing 'jank' and delayed user interactions like typing in a form field.

React engineers realized this architecture couldn't scale for highly interactive apps. They needed to decouple diff calculations from DOM writes and enable pausing for urgent user events. This fundamental need drove the ambitious rewrite to Fiber, which re-engineered React's internals for smoother, more fluid web experiences.