If you’re doing an @world emerge, then you’ll be recompiling all installed packages with updates, including dependencies.

One of the heavier packages that’s included in almost every desktop profile as a dependency somewhere is dev-qt/qtcore (full list of packages in the standard desktop profile here, though each package listed here will have its own dependencies which may have their own dependencies, etc. So it is not an exhaustive list), qtcore also appears to be what was compiling when the photo in your post was taken so is likely the primary cause of that specific long build time.