It's a balance, but on many projects the overhead of the headers themselves is so large that concatenating a few .cpp files together doesn't increase incremental compilation time significantly over simply building each .cpp file in isolation.

Incremental compilation opportunities are rarer than we'd like on C++: as soon as you add a new class member or function, that's potentially a huge recompile.

You can have all the files you're working on not concatenated.

That's painfully manual.

It doesn't need to be. I think it shouldn't be hard to make a build script that re-makes the bundles except for files modified in the last hour.