The part of Discover displaying the loading state is still Angular. This should be converted to React using the [Eui Loading Spinner](https://elastic.github.io/eui/#/display/loading).  This is the part of the Angular template that has to be replaced with a new directive https://github.com/elastic/kibana/blob/8a65950de40a7195652879ca92d9b2cd64c798d5/src/plugins/discover/public/application/angular/discover.html#L62 Here's high level description how to create and register an angular directive in Discover #65326
The part of Discover displaying the loading state is still Angular. This should be converted to React using the Eui Loading Spinner.
This is the part of the Angular template that has to be replaced with a new directive
kibana/src/plugins/discover/public/application/angular/discover.html
Line 62 in 8a65950
Here's high level description how to create and register an angular directive in Discover #65326