Pkg is a lot slower than PetGet to resolve deps on Bionic
.. although they are about the same on Stretch 7.5 CE.
Look into some fixes, tweaks, or snippets from PetGet that can speed up dep resolution..
One solution is to only go 12 or 13 levels deeps, not 20.
Also see the 'Check dependencies installed pkg' program (in Menu->Setup).. It's fast.