The biggest issue cited in this article is a regression in the type checker, which is an open bug that apparently is just lower priority than whatever else the Rust team is working on instead of ensuring compilation time is fast.

Only once that was worked around did they article really start having to pay attention to the code generation and linking stages, and the issues there seem to mostly be about lack of parallelism caused by the "crates" model combined with link time optimizations and incremental compiles being off by default. You don't need to make a fundamentally faster code generator that makes worse output to deal with this: the performance losses you get from parallel, incremental, and separate compilation techniques are so minimal that only the most serious C++ projects go out of their way to combine translation units and activate LTO.