I don't know about this specifically, but Rust in general is fast and at least memory safe (which helps with general security but doesn't guarantee it at by any means).
I also used Brave for quite a while and anecdotally it was quite fast. So in my personal experience the performance is good.
I don't see it as a step towards bring google-like, personally. But who knows with FF these days
I'd love to see .source
Could be used for a lot of different tech and non tech things: open.source, linux.source, lifehacks.source, re.source
Great potential