I don't understand how UB could be faster for large projects.

I imagine those projects for which it works must be very tightly connected indeed. A well-designed piece of software should have as little interdependency as possible, so when a change is made in one or two files only (especially if they're implementation files), there's no need to recompile the entire project. Only those files and their reverse dependencies.

Of course there are some files that are used in a large portion of the project. But they should be the exception rather than the rule. Is this not true?