Zig is starting to migrate to custom backends for debug builds (instead of using LLVM) plus incremental compilation.

All Zig code is built in a single compilation unit and everything is compiled from scratch every time you change something, including all dependencies and all the parts of the stdlib that you use in your project.

So you've been comparing Zig rebuilds that do all the work every time with Rust rebuilds that cache all dependencies.

Once incremental is fully released you will see instant rebuilds.