How does progressive rendering actually that most OS GUIS are not multithreaded ?

The preview in finder is still going to block. Im saying this as a MAC user speaking of experience browsing mounted drives that sometimes are slow and it feels like the window freezes up.

Thumbnailing happens in a background process and shouldn’t block the main thread; rather hangs are usually because fetching xattr over SMB is slow and Finder will block until it’s enumerated the metadata of every file in a folder.

Preview actually does use progressive rendering for large enough images, even from SSDs. You can tell because opening an image will be blurry for several seconds, even when the full size image could have been decoded in a quarter second or less.

Progressive rendering is intended for the Web and it does work.