I was indeed wondering the same: a new render engine seems like a huge and risky investment (even Microsoft gave up on the existing one they have, that says something). If it really is from scratch, it probably just supports only the most popular features, which would cut complexity and give a large performance boost by itself.

Alternatively: Chrome 1.0 was also super fast compared to the competition. And I remember switching from a full browser on Android to my custom browser that was like 10 lines of code around a webview element, which was really an order of magnitude faster. If they modified an existing render engine and wrote their own browser UI, that could also deliver this speed boost.

So this seems relatively doable to me, but you get fewer features.

Let's see if they make this available for a broad audience and if, in five years when they accumulated all the features, it's still faster.