Why is the compiler slow in the first place? I have no rust knowledge, how slow us slow, lets say in comparison to a c compiler?

What is the performance killer?

Doing stuff isn't free. For instance, Go compiles relatively quickly for a modern language, but the biggest reason for that is that it does less stuff than most compilers... less optimization, less checking, and some stuff built into the language to avoid some of the problems with having to read lots of headers just to compile a file and other ways of doing less stuff, but mostly the key is it does less stuff, in both the good and bad senses of that.

If you want something like Rust that offers guarantees and checks and cross-checks by the boatload, it adds up. Macros, monomorphization, implicit code generation with traits and all those other things add up too. And you can't always get O(n) or O(n log n) code to implement those checks. Maybe it can be sped up and maybe there's tricks here or there, but at the Pareto frontier, a language that has more checks will be slower to compile than one that has fewer.

And that's not a bad thing or a deficit in Rust, it's just the nature of the beast.

First of all, the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"[0].

Now, is rustc slower than e.g. clang? by how much? why?

Those are different (and complicated) questions. It really depends on what you're compiling, but I'd say rustc can be 1-5x slower (maybe more at times?).

The reasons are many and varied, but in general rust compilation is slower because the compiler is doing way more things compared to C (monomorphization, complex trait resolution + type inference, borrow checker..)

[0]: Also I'd argue that "slow" without a concrete point of reference is a meaningless term in this context.

Zero-cost abstractions aren't zero cost in compilation time. High-level abstractions translate to a lot of boilerplate that the compiler has to optimize out.

In unoptimized builds often the linker is the bottleneck. Rust/Cargo can parallelize most of the build, generating tons of code and debug info, but then the poor linker has to consume all of it at once. The object/exe formats were designed in ancient times, so they're hard to build incrementally or in parallel (some linkers are trying).

Its similar to C++ compilers, there is no one reason. It is a language that tries to optimize a lot, its a big language, it does safety checks, it uses llvm which is a bit slow, its a language that makes use of generics which generate extra code etc etc.

It's doing static analysis that many other languages don't do at compile time.