Can DUB --combined builds be faster?

[Guillaume Piolat via Digitalmars-d-learn]

I'm cargo-culting the use of --combined with DUB because I 
somehow think inlining will be better in this way. (For thos who 
don't use DUB, what it does is compiling the whole program with a 
single compiler invokation instead of making one static library 
by package.)

But I've never measured much speed-up that way so I wonder if 
it's a dumb thing to do.

Is there a theoretical reason --combined builds may be faster?