The issue - WordPress/gutenberg#6177 is one of the most discussed issues in the Gutenberg project but its discussion is overwhelming and progress has stagnated.
Since the issue was first written in April 2018, the block editor and WordPress have changed and improved but many improvements to improve displaying images remain.
This document summarizes the remaining deficiencies of image loading mentioned in WordPress/gutenberg#6177
At its core (pun intended), the primary issue is: larger than necessary images (by pixel size (dimensions)) and as well as higher resolution than necessary, are loaded for an end user when they visit a page with images.
This behavior occurs when WordPress' and a theme's usual defaults (e.g. no alignment specificed, default image size created) are used.