[React 18] Add slow mode

[gaearon]

This is an essential part that most comparisons and ports to other libraries tend to miss.

Of course, React is not the most efficient way to render a viz like this. You're better off with GL. But this particular example I'm adding has a neat aspect.

When the checkbox is ticked, each square is forced to spend 0.1ms blocking the thread. This simulates "death by a thousand cuts" where you have a large update that needs to run a bunch of user-written code. There's just no way you can "optimize it away". What's interesting is how the system handles a bunch of computational work distributed between a tree of components.

Compare the behavior with startTransition vs without it.

Demo link

Tick this checkbox:

The effect is pronounced after you increase the tree size with the slider on the left.

[vercel]

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🔍 Inspect: https://vercel.com/swizec/react-fractals/7HkcdkzhmX6twbAFVQKjAx3eA2F7
✅ Preview: https://react-fractals-git-fork-gaearon-block-swizec.vercel.app