This file is the entire source of the component: https://github.com/zzarcon/react-circle/blob/57bb4d19df08b73...

A lot of the work seems to be the React diffing? The only thing that changes between frames when dragging the slider is the style attribute.

Furthermore, the author seems to have included the development version of React. And the author used development settings for the webpack compilation. The compiled JS is full of eval() calls.