SlateJS: First time rendering

[mkoskim]

Rendering the editor view takes a lot of time. I examined it a bit, and the root reason seems to be the React rendering. What SlateJS does is that it creates a React component from everything, including the text fragments. When you have lots of small fragments (scenes, paragraphs and so on), it results to large number of React components, and it takes a time to render it.

When creating release builds this eases a bit, as the build is somewhat optimized.

There is not much to do if I keep using SlateJS. So, at some point, we might need to consider changing the underlaying editor component to something that performs better with large text blocks. I know there exists some, but most of them can not handle nested structures. With those, you need to have mechanism to "flatten" the nested structure to editor component, and "re-nest" it back when reading the content from editor.