As far as I'm aware, using the self-hosted backend for anything other than Debug mode is a goal, but a far-future goal.

I believe the most relevant links are https://github.com/ziglang/zig/issues/16270 and https://github.com/orgs/ziglang/projects/2/views/1?pane=issu... (as you can see, nothing is concrete yet, just vague mentions of optimization passes)