The reason to care about compile time is because it affects your iteration speed. You can iterate much faster on a program that takes 1 second to compile vs 1 minute.

Time complexity may be O(lines), but a compiler can be faster or slower based on how long it takes. And for incremental updates, compilers can do significantly better than O(lines).

In debug mode, zig uses llvm with no optimization passes. On linux x86_64, it uses its own native backend. This backend can be significantly faster to compile (2x or more) than llvm.

Zig's own native backend is designed for incremental compilation. This means, after the initial build, there will be very little work that has to be done for the next emit. It needs to rebuild the affected function, potentially rebuild other functions which depend on it, and then directly update the one part of the output binary that changed. This will be significantly faster than O(n) for edits.