Thanks for getting back to me Dave.
I understand what you're saying about collection images not getting responsive variants yet, but something is definitely modifying them after upload.
All the images I was uploading to collections were at 2x resolution, each with @2x appended to the end of the filename. When inspecting the image on the site, the resolution had been cut in half.. So they're at least being resized to 1x resolution, and I'm betting that's where the nasty compression is coming from.
I just did a test by uploading a 1x resolution image and sure enough there's no compression. The image does look better, but unfortunately it's not 2x so it's not sharp on retina screens.
—and WOAH I just fixed it! I just reuploaded the 2x image, but after removing the @2x from the filename, and voila, NO COMPRESSION, and it retains it's original retina resolution!
So, there it is. @2x on the filename makes for some really lossy compression on Webflow's end. Sorry if this is a known thing, but it's news to me. Guess I'm renaming all my images and reuploading.
LMK if you discover anything else on your end Dave, and thanks again for checking it out on the holiday.