I was reading your post about the impact of removing srcset from mobile pageviews, and I was impressed by the amount that it reduced the image bandwidth. Do you think the reduction in 2G load time is due to devices that are running on 2G that were previously requesting the 2x version of the image, but now are forced to request the 1x version? Or is there some other explanation? I know that at least back in 2015-16 there were some bugs in browser engines that would request both the "src" image and also one of the "srcset" entries as well, although I think those have generally been fixed.
I'm also curious whether those graphs might have overestimated the effect, due to the issue Brion brought up on T134115#2257578 (sorry, AbuseFilter is blocking me from making that into a link!) which would have caused the non-srcset versions of pages to sometimes display to desktop users as well. It's a bit weird though because I think then (and even now) the majority of "retina" displays were mobile.
Also wondering whether most of those gains would have been eaten up by Reading/Web/Lazy loading of images on all wikis anyway, but that's probably harder to answer after-the-fact.
I'm mainly asking because naively, it seems like this change (plus Brion's addition to the parser cache key) roughly doubles the number of parser cache misses, because now mobile and desktop pages need to use entirely separate cache entries. Am I missing something that makes it not that way, or makes that less of a problem than I (not being associated with WMF at all) would think?