First, that's uncompiled/jitted. Second: the 80/20 rule.

There are surely to be diminishing returns for sure. Ladybird is clearly improving this fast right now because the devs are picking the low hanging fruit. But we also don't need 100% parity before Ladybird usable. And when users pick it up then it begets more donations and more devs resources, which mean more improvement. So there is reason to be optimistic.

That plus a few of the team have been deeply involved in other browsers. This is nothing new for them.

Doesn't make it certain, but it is a good foundation to be working from.