I use a 6 year old desktop which wasn't that great even when I built it, and a pretty terrible 8 year old laptop, and I don't have any problems with uBlock Origin's performance. I have almost every filter list enabled (except for some regional ones), which results in 153486 network and 173646 cosmetic filters total.

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.