LTO can be parallelized, both implicitly from the Makefile, and also within the compiler. GCC's -flto itself takes an optional argument to control the number of parallel threads/jobs. See also the -flto-partition option for selecting symbol partitioning strategies.

Rust/Cargo does this automagically, except the only control you have are the crate and module boundaries. The analogous approach for C is to (optionally) manually group compilation units into a smaller set of object files in your Makefile, LTO'ing each object file in parallel (make -j), and then (optionally) telling the compiler to partition and parallelize a second time on the backend. Which is what Rust does, basically, IIUC--a crate is nominally the LTO codegen unit, except to speed up compilation Rust has heuristics for partitioning crates internally for parallel LTO.