I don't understand this way of thinking. Rust isn't magically fast. You can use most languages to write both performant and lazy code.

Also just so you know, Brave isn't "written" in Rust alone, it is a big software with a lot of parts, including but not limited to a rendering engine, a JS VM and a WASM engine.

The Rust part at most (unconfirmed) would be the glue that connects them together, and I doubt that's where the bottleneck is for most browsers.