Boarding speed is constrained by aisle space competition. The most efficient (fastest) boarding methods enable people in different parts of the plane to board at the same time (i.e, boarding a mix of some people in the front, middle, and back of the plane at the same time would be fastest). Delta is moving to an approach that further increases aisle space competition by boarding the exact same parts of the plane together.

First: Boards the first portion of the plane
C+: Board next front portion of the plane
Gold/Plat: Board primarily preferred seats in the front of the plane
Main Cabin 1 (Silver, CC holders): Board primarily preferred seats in the front portion of the plane
Main Cabin 2/3: Board the middle portion of the plane
Basic Economy: Board back of the plane and/or middle seats

At least before flights with high density of Elites (Gold/Plats) got to board at the same time as C+ - meaning that there was some heterogenous boarding going on (preferred seats + C+ seats).

The flights this will slow boarding down the most are the flights you mention that have 70+ people in the SKY lane.

One can argue that it won't make that much of a difference - but that is only because Delta already has the (statistically) slowest boarding process of any US carrier. So maybe it is just an extra minute or two, but it is an extra minute or two added to what is already the slowest boarding process.