arena allocation is a big part of it, but also oxc benefits from not having to support the same breadth of legacy transforms that swc accumulated over time. swc has a lot of surface area from being the go-to babel replacement -- oxc could design the AST shape from scratch with allocation patterns in mind. the self-hosting trap (writing js tooling in js) set a performance ceiling for so long that when you finally drop down to Rust and rethink the data layout, the gains feel almost unfair