[1]: https://learn.microsoft.com/en-gb/cpp/build/reference/gl-who...
[2]: https://learn.microsoft.com/en-gb/cpp/build/reference/ltcg-l...
These two options also control profile-guided optimisation.
MSVC's LTCG (LTO) pipeline has been heavily in use at Microsoft for decades now. PGO and LTCG go together. LTCG is the default way most of our binaries are built.
A lot of work has been done to make MSVC LTCG fast and scalable. In particular, the compiler backend is multithreaded on a per-function basis, and there is also support for incremental LTCG that reuses previously compiled machine code and summary information.