When it compiles while you're using it and while you sleep/eat/go out/etc, no time is lost. Watts maybe.
Only new Gentoo users sit and watch gcc output fly by in a console window. It can be mesmerizing like the older defragmenter on Windows. But once that gets boring they don't waste time just waiting for it.
That maybe fine when your doing your initial install but I don't know too many Gentoo guys that when they need a package that put it on a list to compile while away.