Cite a source? This isn't my experience at all, Rust is still way faster at compilation than C++ on average. The advantage that C++ currently has is ccache which helps avoid needless rebuilds (which pairs well with C++'s smaller compilation units relative to Rust); building this behavior into the compiler is part of what the incremental compilation initiative is addressing.
> It consists of ~7700 lines of Rust
...
> Compile times with Rust are Not Great. This is easily my single biggest gripe about Rust right now. The build for A Snake’s Tale takes 15+ seconds, which makes iterating rather tedious. The current incremental compiler work also doesn’t seem to make the build for A Snake’s Tale’s codebase any faster.
Actually, the compiler isn't the bottleneck - most of the time is LLVM codegen. So 'cargo check' is fast - the miri MIR interpreter likewise.