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.