iTerm2 has a new drawing engine that uses Metal 2
There's currently a bug with multi-GPU based MBPs and the new metal renderer.  The new metal renderer causes it to always use high performance graphics whether plugged in or not.  I had to debug why my battery was dying faster than usual and traced it back to iTerm2 and the metal renderer.The developer was working on the fix, but I have not seen a fix announced in the release notes yet.