The time spent by emerge is utterly dwarfed by the time spent to build the packages, so who cares? Maybe it's different if installing a binary system but don't think most people are doing that.

If you can emerge in 2.86s user you can do it right before you emerge world, meaning it's all "done in one interaction" (even if the actual emerge takes an hour - you don't have to look at it.

Whereas if emerge is taking 5-10 minutes, you have to remember to come back to it, or script it.

When using multiple overlays, emerge-webrsync is ungodly slower compared to git.