Real-World Effectiveness of Brotli: If enabling Brotli is as simple as flicking a checkbox in the admin panel of your CDN, please go ahead and do it right now
... if you haven't or can't enable Brotli, you're not missing out on much.
Wrap your description in CDATA comments and insert the images as HTML img tags.
... enter the new image URLs in the feed ... minimum 1400 x 1400 px, maximum 3000 x 3000 px, JPG or PNG, RGB, and file size less than 500kB. Note however that the episode images which appear in small form in at the top of iTunes (on a computer) when playing (and in large form when that is clicked) have to be actually embedded in the media file - please see this page. The images in the 'itunes:image' tags in the feed don't appear when subscribing in iTunes, and only in the Search results in the iTunes Store, though they may appear in the Podcasts app ...
chromestatus.com
: HTML & JavaScript usage > all features > timeline: suggesting that in May 2020 ~2.3% of (sampled) pages use loading=lazy
on at least one image (from zero a year before), cf 0.25% for eager. These days [2018-11-05] most devices have a device-pixel-ratio between 1.0 and 4.0. ... This ratio doesn't have to be a whole number. 1.5, 2.4, and 2.5 are all device-pixel-ratios of common devices.