I find that projects up to around 50K lines compiles instantly as a single file/"unity build" and is my preferred way for most of my projects.
It's important to stress that it does more than reduce link time: unity builds completely eliminate linking when plugging together submodules, and the final project linking simply has far fewer object files to link.
I find that projects up to around 50K lines compiles instantly as a single file/"unity build" and is my preferred way for most of my projects.