[mikepurvis]

Running a unity build [1] means the compiler only has to parse and reason about the headers once. So you lose the parallelism of multiple TUs, but you gain on not doing the header crunching over and over again. Whether your codebase will benefit from that is a matter for testing, but many do.

[1]: https://en.wikipedia.org/wiki/Unity_build

[behiri]

also by using unity build you don't need to link object files. I think that is main reason to use unity build, it eliminates need for any build system and compiling source code in a new machine is as simple as compiling a single transition unit.

[maccard]

Unreal is far too big for one unity blob. It still requires a build system for many other reasons.